ELECTRICAL  NATURE 

of 
MATTER  AND  RADIOACTIVITY 

Harra  C.Jones 


ft 


• 


THE 

ELECTRICAL    NATURE 


OF 


MATTER   AND    RADIOACTIVITY 


BY. 

HARRY  C.  JONES 

PROFESSOR  OF  PHYSICAL  CHEMISTRY  IN  THE 
JOHNS  HOPKINS  UNIVERSITY 


THIRD  EDITION  —  COMPLETELY  REVISED 


NEW    YORK 

D.  VAN  NOSTRAND  COMPANY 

25  PARK  PLACE 

1915 


V 


Copyright,  1906,  by 
D.  VAN  NOSTRAND  COMPANY 

Copyright,  igio,  by 
D.  VAN  NOSTRAND  COMPANY 

Copyright,  1915,  by 
D.  VAN  NOSTRAND  COMPANY 


PREFACE  TO  THE  FIRST  EDITION 

THE  content  of  this  book  has  already  been  published 
as  a  series  of  articles  in  the  Electrical  Review.  The  two 
correlated  subjects  under  consideration  are  of  such  general 
interest  that  it  has  seemed  desirable  that  the  discussion  of 
them  should  be  made  accessible  in  compact  form. 

The  several  chapters  as  they  originally  appeared  have, 
therefore,  been  carefully  revised  and  brought  together  in 
one  volume.  The  author  would  extend  his  sincere  thanks 
to  his  assistant,  Dr.  H.  S.  Uhler,  for  a  number  of  valuable 
suggestions  in  connection  with  the  revision  of  the  work. 

The  aim  of  the  writer  has  been  to  present  the  more  im- 
portant facts  and  conclusions  in  connection  with  the  work 
on  the  "  Electrical  Nature  of  Matter  and  Radioactivity,"  as 
far  as  possible  in  non-mathematical  language.  This  has 
been  done  with  the  belief  that  there  are  a  large  number  of 
those  who  have  a  truly  scientific  interest  in  these  most 
recent  and  important  developments  in  Physics  and  Physical 
Chemistry,  but  to  whom  a  more  technical  and  rigidly  math- 
ematical treatment  might  not  appeal.  To  all  who  desire 
such  a  treatment,  the  admirable  books  by.  Thomson,  on  the 
"  Conductivity  of  Electricity  through  Gases,"  and  by  Ruther- 
ford, on  "Radioactivity,"  are  heartily  recommended. 

While  this  work  is  written  in  a  semi-popular  style,  the 
attempt  has  been  made  to  treat  the  subject  with  scientific 
accuracy.  The  facts  presented  have  nearly  always  been 
taken  directly  from  the  original  sources.  Since,  however, 
this  is  a  comparatively  elementary  discussion,  references  to 
the  original  papers  are  given  chiefly  in  the  cases  of  the  more 
important  contributions.  All  of  those  who  desire  to  go 

iii 

346539 


IV  PREFACE 

more  fully  into  these  subjects  are  urged  to  read  as  many 
as  possible  of  the  original  articles. 

If  this  little  book  should  contribute  even  in  a  small  meas- 
ure towards  supplying  the  general  demand  for  knowledge 
in  the  field  which  it  covers,  it  will  more  than  repay  for  the 
time  and  labor  that  have  been  spent  in  its  preparation. 

HARRY  C.  JONES. 

PREFACE  TO  THE  SECOND   EDITION 

THE  aim  in  preparing  a  second  edition  of  this  work  is 
to  bring  it  up  to  date  as  far  as  matters  of  fundamental 
importance  are  concerned.  Most  of  the  epoch-making  dis- 
coveries in  connection  with  radioactivity  were  made  in 
the  earlier  stages  of  the  work,  but  many  important  con- 
tributions to  our  knowledge  in  this  field  have  been  pub- 
lished in  the  last  few  years.  This  material,  as  far  as  is 
consistent  with  the  scope  and  size  of  this  book,  has  been 
incorporated  in  this  edition. 

The  author  gladly  accepts  this  opportunity  to  express 
his  '^anks  to  his  assistant,  Dr.  W.  W.  Strong,  for  valuable 
aid  in  revising  this  book. 

xl.    \^.   J. 

JOHNS  HOPKINS  UNIVERSITY,  BALTIMORE 
June,  1910. 

PREFACE  TO  THE  THIRD  EDITION 

IN  preparing  the  third  edition  of  this  little  work,  some 
minor  changes,  and  some  additions  at  the  ends  of  the 
chapters  have  been  made.  It  is  gratifying  to  see  how 
quickly  the  third  edition  of  this  book  has  been  called  for. 

The  author  takes  pleasure  in  expressing  his  thanks  to 
his  future  coworker,  Mr.  Edward  O.  Hulburt,  for  valuable 
suggestions  in  connection  with  the  work  of  revision. 

February,  1915.  H.  C.  J. 


CONTENTS 

CHAPTER  I 

PAGE 

THE  ELECTRICAL  CONDUCTIVITY  OF  GASES i 

Conditions  which  increase  the  conductivity  of  gases.  How  a 
conducting  gas  differs  from  a  non-conducting.  The  ratio  of  the 
charge  to  the  mass  of  the  ion  in  a  gas.  The  cathode  ray.  The 

value  of  —  for  the  cathode  particle.     The  ratio  —  constant  for 
m  m 

different  gases.     The  ratio  —  varies  for  the   different   ions   of 
wt 

electrolytes.     The  value  of  —  for  gaseous  ions  produced  by  differ- 

ftt 

ent  means. 

CHAPTER  II 

THE  DETERMINATION  OF  THE  MASS  OF  THE  NEGATIVE  ION  IN  GASES  .     10 

Work  of  J.  J.  Thomson.  Comparison  of  the  charge  on  a 
gaseous  ion  with  that  on  a  univalent  ion  of  an  electrolyte.  The 
ratio  of  the  charge  to  the  mass  for  the  positive  ion. 

CHAPTER  III 

NATURE  OF  THE  CORPUSCLE.    THE  ELECTRICAL  THEORY  OF  MATTER      19 

Work  of  Thomson  and  Kaufmann.  The  electron  the  ulti- 
mate unit  of  matter.  Earlier  attempts  to  unify  matter.  Other 
relations  between  the  elements. 

CHAPTER  IV 

THE  NATURE  OF  THE  ATOM  IN  TERMS  OF  THE  ELECTRON  THEORY     .       29 

Thomson's  conception  of  the  atom.  The  electron  theory  and 
the  Periodic  System.  The  atom  in  terms  of  the  electron  theory. 
Cations  and  anions  in  terms  of  the  electron  theory.  The  mass 
of  an  ion  not  exactly  the  same  as  that  of  the  atom  from  which  it 
is  formed.  The  electron  theory  and  radioactivity.  More  recent 
view  as  to  the  nature  of  the  atom. 

CHAPTER  V 
THE  X-RAYS 41 

Nature  of  the  X-ray.  The  Becquerel  ray.  Properties  of  the 
Becquerel  ray.  The  thorium  radiation.  Recent  work  on  the 
nature  of  the  X-ray. 


vi  CONTENTS 

CHAPTER  VI 

PAGE 

THE  DISCOVERY  OF  RADIUM 49 

The  separation  of  radium  from  pitchblende.  The  spectrum  of 
radium.  The  atomic  weight  of  radium. 

CHAPTER  VII 

OTHER  RADIOACTIVE  SUBSTANCES  IN  PITCHBLENDE 63 

Polonium.  Actinium.  The  more  important  methods  used 
in  studying  radioactivity.  Properties  of  the  radiations  given  out 
by  radioactive  substances. 

CHAPTER  VIII 
THE  ALPHA  RAYS 72 

The  ratio  —  for  the  alpha  particle.    The  mass  of  the  alpha  par- 

m 

tide.  Critical  velocity  of  the  alpha  particles.  Alpha  particles 
produce  delta  particles.  Alpha  particles  are  probably  helium 
atoms.  Action  of  the  alpha  particles  on  a  photographic  and  on 
a  fluorescent  plate.  Stopping  the  spinthariscope  power  of  mat- 
ter for  the  alpha  particles. 

CHAPTER  IX 

THE  BETA  AND  GAMMA  RAYS 85 

The  beta  rays.     Nature  of  the  charge  carried  by  the  beta  par- 
ticles.   The  determination  of  —  for  the  beta  particle.     The  mass 
m 

of  the  beta  particle.  Relation  to  the  cathode  particle.  Cathode 
rays.  Beta  rays  from  radium.  The  gamma  rays.  Secondary 
radiations  produced  by  beta  rays.  Summary  of  the  properties  of 
the  alpha,  beta,  and  gamma  rays.  Total  number  of  particles 
shot  off  by  radium. 

CHAPTER  X 

OTHER  PROPERTIES  OF  THE  RADIATIONS 98 

The  self-luminosity  of  radium  compounds.  Phosphorescence 
produced  by  radium  salts.  Radium  increases  the  conductivity 
of  dielectrics.  Chemical  effects  produced  by  radioactive  sub- 
stances. Physiological  action  of  the  radiations  from  radium. 

CHAPTER  XI 

PRODUCTION  OF  HEAT  BY  RADIUM  SALTS 106 

Measurement  of  the  heat  liberated  by  salts  of  radium.  Method 
of  the  Bunsen  ice  calorimeter.  Results  of  heat  measurements. 
Source  of  the  heat.  Effect  on  solar  heat.  Does  radium  exist  in 
the  sun?  Terrestrial  heat  produced  by  radium.  Bearing  on  the 
calculated  age  of  the  earth.  Theories  as  to  the  source  of  the  heat 
produced  by  radium.  Calculation  of  the  amount  of  heat  liberated 
by  radium  on  the  above  theory  that  the  heat  is  produced  by  the 
8  particles.  Three  remarkable  properties  of  radium. 


CONTENTS  Vll 

CHAPTER  XII 

PAGE 

EMANATION  FROM  RADIOACTIVE  SUBSTANCES 118 

Discovery  of  the  thorium  emanation  by  Rutherford.  Method 
of  obtaining  the  emanation.  Amount  of  the  emanation.  Nature 
of  the  emanation.  Diffusion  of  the  emanation.  Approximate 
determination  of  its  molecular  weight. 

CHAPTER  XIII 

HELIUM  PRODUCED  FROM  THE  EMANATION 127 

Recovery  of  emanating  power.  Decay  of  emanation.  Heat 
evolved  by  the  emanation.  Helium  produced  from  the  emana- 
tion. This  is  not  a  tramsmutation  of  the  elements.  Further 
experiments  on  the  production  of  helium  from  radium.  Relation 
between  the  emanation  and  helium.  Some  remarkable  results 
obtained  by  the  action  of  the  radium  emanation. 

CHAPTER  XIV 

INDUCED  RADIOACTIVITY 140 

Induced  radioactivity  produced  by  the  emanation.  Induced 
radioactivity  undergoes  decay.  Induced  radioactivity  due  to  the 
deposit  of  radioactive  matter.  Properties  of  the  radioactive 
matter  deposited  by  the  emanation  from  radioactive  substances. 
Emanation  X.  Facts  that  must  be  taken  into  account  in  dealing 
with  the  decay  of  induced  or  excited  radioactivity.  Radium 
probably  identical  with  polonium.  Summary  of  the  decomposi- 
tion products  of  radium.  Decomposition  products  of  other  radio- 
active substances.  Interpretation  of  these  facts.  Radiothorium — 
a  new  radioactive  element.  Decomposition  products  of  activium. 

CHAPTER  XV 

PRODUCTION  OF  RADIOACTIVE  MATTER    .     .     .     ^    .     .^   .     .     .     161 

Continuous  formation  of  radioactive  matter  in  uranium.  Re- 
covery of  activity  by  uranium,  and  decay  of  activity  in  uranium  X. 
Radiation  from  uranium  X.  Continuous  formation  of  radio- 
active matter  from  thorium.  Properties  of  thorium  X.  Decay 
of  its  radioactivity.  Thorium  X  produces  the  thorium  emana- 
tion. Recovery  of  radioactivity  by  thorium.  Rate  at  which 
thorium  recovers  radioactivity  independent  of  conditions.  Ra- 
dium does  not  give  rise  to  substances  corresponding  to  uranium 
.  X  and  thorium  X. 

CHAPTER  XVI 

THEORETICAL  CONSIDERATIONS 170 

Importance  of  a  theory  or  generalization.  The  more  important 
facts  in  connection  with  uranium.  The  more  important  facts  in 
connection  with  thorium.  The  more  important  facts  in  connection 
with  radium.  Theory  of  Rutherford  and  Soddy  to  account  for 
radioactive  phenomena.  The  transformations  of  the  radioactive 
elements  differ  fundamentally  from  ordinary  chemical  reactions. 
The  electron  theory  of  J.  J.  Thomson  as  applied  to  radioactivity. 
Is  matter  in  general  undergoing  transformation? 


Vlll  CONTENTS 

CHAPTER  XVII 

WIDE  DISTRIBUTION  OF  RADIOACTIVE  MATTER  AND  THE  ORIGIN  OF 

RADIUM 183 

Radioactive  matter  in  the  earth  and  sea.     Radioactive  matter 
in  the  air.     Is  matter  in  general  radioactive?    The  origin  of 
radium.     Ionium.     The   complete  series  of  transformations  in 
which  radium  is  involved.     Emanium.     Conclusion. 
ATOMIC  WEIGHTS  OF  RADIOACTIVE  LEAD  FROM  DIFFERENT  SOURCES    204 


ABBREVIATIONS  OF  THE  TITLES  OF 
JOURNALS 

Amer.  Chem.  Journ.  =  American  Chemical  Journal. 

Amer.  Journ.  Sci.  =  American  Journal  of  Science. 

Ann.  Chim.  Phys.  =  Annales  de  Chimie  et  de  Physique. 

Ann.  d.  Phys.  =  Annalen  der  Physik  (Drude). 

Ber.  d.  deutsch.  chem.  Gesell.  =  Berichte  der  deutschen  chemischen 

Gesellschaft. 
Cam.  Phil.  Soc.  Proc.  =  Proceeding  of  the  Cambridge  Philosophical 

Society. 

Chem.  News  =  Chemical  News. 
Compt.  rend.  =  Comptes  rendus. 

Journ.  Chem.  Soc.  =  Journal  of  the  Chemical  Society  of  London. 
Journ.  de  Chim.  Phys.  =  Journal  de  Chimie  Physique. 
Nat.  =  Nature. 

Phil.  Mag.  =  Philosophical  Magazine. 

Phil.  Trans.  =  Philosophical  Transactions  of  the  Royal  Society. 
Phys.  Rev.  =  Physical  Review. 
Phys.  Zeit.  =  Physikalische  Zeitschrift. 
Roy.  Soc.  Proc.  =  Proceedings  of  the  Royal  Society. 
Wied.  Ann.  =  Wiedemann's  Annalen. 
Zeit.  phys.  Chem.  =  Zeitschrift  fur  physikalische  Chemie. 


The    Electrical  Nature  of  Matter  and 
Radioactivity 

CHAPTER  I 

THE  ELECTRICAL  CONDUCTIVITY  OF  GASES 

THE  power  of  gases,  under  normal  pressure  and  at  ordi- 
nary temperatures,  to  conduct  electricity  is  so  small  that  it 
has  been  doubted  whether  pure,  dust-free  gases  can  con- 
duct at  all.  Recent  refined  experiments,  however,  show 
that  while  pure,  dust-free  gases  have  only  a  small  con- 
ductivity, they  have  a  definite  power  to  conduct  electricity, 
which  is  measurable. 

CONDITIONS  WHICH  INCREASE  THE  CONDUCTIVITY  OF  GASES 

While  gases  under  normal  conditions  have  only  slight 
conductivity,  and  are  fairly  good  insulators,  it  is  not  a 
difficult  matter  to  increase  greatly  the  conductivity  of  gases. 
This  can  be  done  in  a  number  of  ways.  When  gases  are 
heated  to  high  temperatures  their  electrical  conductivity 
is  greatly  increased.  According  to  Becquerel,  when  air 
is  heated  to  a  white  heat,  electricity  will  pass  through  it 
when  the  difference  in  potential  is  small.  It  is  also  known 
that  gases  in  contact  with  incandescent  solids  have  their 
conductivity  increased.  Some  interesting  and  important 
facts,  which  it  would  lead  us  too  far  at  present  to  discuss, 
have  been  brought  to  light  through  the  study  of  these 

phenomena. 

i 


2         THE  ELECTRICAL  NATURE  OF  MATTER 

Gases  taken  from  flames  have  been  found  to  show  con- 
siderable conductivity,  which  is  retained  for  some  time 
after  the  gas  has  been  removed  from  the  flame  and  cooled 
down. 

Other  agents  which  increase  the  conductivity  of  gases 
are  Rontgen  rays,  the  presence  of  radioactive  substances, 
and  cathode  rays.  As  these  will  be  taken  up  later  in  some 
detail,  they  will  not  be  discussed  further  in  the  present 
connection. 

HOW  A  CONDUCTING  GAS  DIFFERS  FROM  A  NON-CONDUCTING 

We  have  seen  that  a  gas  in  the  normal  condition  has 
very  small  power  to  conduct  electricity. 

We  have  also  seen  that  the  conducting  power  of  a  gas 
can  be  greatly  increased  by  a  number  of  widely  different 
agents.  The  question  that  would  naturally  arise  in  this 
connection  is,  how  does  a  conducting  gas  differ  from  a 
non-conducting  or  normal  gas?  (We  may  term  a  normal 
gas  non-conducting,  since  its  conductivity  is  so  slight.) 

To  answer  this  question  we  must  study  the  properties 
of  a  conducting  gas,  and  compare  them  with  the  properties 
of  a  non-conducting  gas. 

If  the  conducting  gas  is  made  to  pass  through  a  plug  of 
glass-wool,  or  is  drawn  through  water,  it  loses  its  conduct- 
ing power.  The  conducting  power  of  a  gas  is  also  removed 
by  passing  the  gas  through  a  metal  tube  of  very  fine  bore; 
the  finer  the  bore  the  more  rapidly  the  conductivity  is 
lost. 

The  removal  of  the  conducting  power  by  filtering  through 
glass-wool  shows  that  the  conductivity  of  the  gas  is  due  to 
some  constituent  which  is  filtered  out  mechanically  by  the 
glass-wool.  The  experiments  with  the  metal  tube  show 
that  this  constituent  which  can  be  filtered  out  by  glass- 


THE  ELECTRICAL  CONDUCTIVITY  OF  GASES  3 

wool  is  charged  with  electricity.  These  charged  particles 
in  a  conducting  gas  are  known  as  ions.  Some  of  these 
particles  are  charged  positively  and  others  negatively. 
Since  a  conducting  gas  shows  neither  an  excess  of  positive 
nor  of  negative  electricity  it  is,  as  we  say,  electrically  neutral. 

THE  RATIO  OF  THE  CHARGE  TO  THE  MASS  OF  THE  ION  IN  A 

GAS 

When  an  acid,  base,  or  salt  is  dissolved  in  water,  we  know 
that  it  breaks  down  into  charged  parts  called  ions.  Every 
molecule  of  an  electrolyte  yields  an  equivalent  number 
of  posivitely  charged  parts  or  cations,  and  negatively 
charged  parts  or  anions.  The  ratio  of  the  charge  carried  by 
these  ions  to  their  mass  has  been  determined.  In  the  case  of 
the  hydrogen  ion,  which  is  the  characteristic  ion  of  all  acids, 
it  has  been  found  to  be  of  the  order  of  magnitude  of  io4. 

It  was  recognized  to  be  of  importance  to  determine  the 
ratio  of  the  charge  to  the  mass  of  the  ion  in  gases.  If  we 
represent  the  charge  carried  by  the  gaseous  ion  by  e,  and 

the  mass  of  the  ion  by  m,  the  ratio  in  question  is  — . 

m 

We  shall  take  up  first  the  determination  of  the  ratio  — 

m 

for  the  cathode  particle. 

THE  CATHODE  RAY 

When  an  electric  discharge  is  passed  through  a  high- 
vacuum  tube,  rays  are  sent  out  from  the  cathode  which 
generally  produce  a  greenish  yellow  phosphorescence  where 
they  fall  upon  the  glass  walls  of  the  tube.  These  are  known 
as  the  cathode  rays.  The  nature  of  these  rays  was  for  some 
time  in  doubt.  It  was  thought  by  some  investigators 
that  they  were  waves  in  the  ether.  It  remained  for  Sir  William 


4  THE  ELECTRICAL  NATURE  OF  MATTER 

Crookes  to  give  us  the  accepted  explanation  of  the  nature 
of  the  cathode  rays.  According  to  Crookes  the  cathode 
rays  are  charged  particles,  sent  off  from  the  cathode  with 
very  high  velocity.  They  move  towards  the  anode  in  a 
direction  at  right  angles  to  the  surface  of  the  cathode.  The 
properties  of  the  cathode  rays,  in  general,  are  in  accord 
with  this  theory.  The  cathode  rays  can  be  deflected  by  a 
magnet. 

A  solid  body  placed  in  their  path  casts  a  well-defined 
shadow. 

Cathode  rays  can  probably  produce  certain  chemical 
changes,  especially  of  a  reducing  nature. 

Mechanical  effects  can  readily  be  produced  by  the  cathode 
rays,  as  was  shown  by  Sir  William  Crookes.  A  glass  paddle- 
wheel  is  easily  made  to  move  along  level  glass  tracks  within 
the  tube,  by  allowing  the  cathode  rays  to  impinge  upon 
the  vanes. 

Thermal  effects  are  readily  produced  by  the  cathode 
rays.  By  suitably  concentrating  them  upon  platinum, 
the  metal  is  rendered  incandescent.  All  of  these  facts 
accord  with  the  theory  as  to  the  nature  of  the  cathode  rays, 
advanced  by  Sir  William  Crookes. 

The  discovery  that  cathode  rays  can  pass  through  thin 
films  of  metal  seemed  at  first  to  argue  against  the  Crookes 
theory.  When  we  become  familiar  later  with  the  exact 
nature  of  the  cathode  particle  itself,  we  shall  see  that  this 
argument  is  without  foundation. 

We  shall,  then,  at  present  accept  the  Crookes  theory, 
and  regard  the  cathode  rays  as  consisting  of  negatively 
electrified  particles,  moving  with  high  velocities,  in  straight 
lines  at  right  angles  to  the  surface  of  the  cathode. 

In  the  light  of  the  above  theory  and  the  facts  upon  which 
it  is  based,  we  shall  now  take  up  the  work  of  J.  J.  Thorn- 


THE  ELECTRICAL  CONDUCTIVITY  OF  GASES 


son,  by  which  he  determined  the  value  of  —  for  the  cathode 

m 

particle. 


e 

THE  VALUE  OF  —  FOR  THE  CATHODE  PARTICLE 

m 

The  value  of  —  for  the  cathode  particle  was  determined 
m 

by  J.  J.  Thomson,1  as  follows:  The  cathode  is  placed  near 
one  end  of  an  exhausted  tube,  and  the  anode  removed  only 
a  short  distance  from  the  cathode.  Beyond  the  anode  on 
the  side  removed  from  the  cathode  is  placed  a  metal  plug 
connected  with  the  earth.  A  small  hole  is  bored  through  the 
centre  of  the  anode  and  the  metal  plug.  Cathode  rays  pass 
through  these  holes  and  fall  on  the  wall  of  the  vacuum  tube 
at  the  end  of  the  tube  farthest  removed  from  the  cathode. 
Since  the  holes  in  the  metal  plates  are  small,  we  have  a  nar- 
row beam  of  cathode  rays  striking  the  inner  wall  of  the  glass 
vessel,  forming  a  small,  phosphorescent  spot  on  the  glass. 
We  have  seen  that  the  cathode  rays  are  deviated  by  a 
magnetic  field.  If  the  whole  tube  is  now  properly  placed 
in  a  magnetic  field,  the  path  of  the  cathode  particles  will 
be  changed,  and  they  will  impinge  upon  the  glass  wall  at 
some  point  different  from  that  which  they  originally  bom- 
barded when  no  magnetic  field  was  present.  Measuring 
the  magnitude  of  this  deflection  we  can  calculate  the  value 

a 

of  — ,  in  which  v  is  the  velocity  of  the  ion. 

vm 

We  have  thus  determined  the  ratio  of  e  to  vm. 

We  must  now  determine  the  value  of  v  in  order  to  obtain 

the  ratio  — . 
m 

Into   the   above-mentioned   vacuum   tube   are   inserted 

' Phil.  Mag.,  44,  293  (1897). 


6          THE  ELECTRICAL  NATURE  OF  MATTER 

two  parallel  plates  of  aluminium,  which  are  so  arranged 
that  the  beam  of  cathode  particles  passes  between  them. 
The  plates  are  also  parallel  to  the  original,  undeflected 
beam.  These  metal  plates  are  attached  to  some  electrical 
source,  and  maintained  at  a  known  difference  in  potential. 
We  thus  have  between  the  plates  an  electric  field.  The 
electrostatic  intensity  s,  due  to  this  field,  deflects  the  ion 
with  a  force  se,  e  being  the  charge  upon  the  ion.  The  force 
due  to  the  magnetic  field  already  considered  is  fev,  f  being 
the  strength  of  the  field,  e  the  charge  carried  by  the  ion, 
and  v  the  velocity  of  the  ion. 

By  suitably  charging  the  metal  plates,  the  electrical  and 
magnetic  forces  can  be  made  to  act  counter  to  one  another. 
These  two  counter  forces  can  readily  be  made  equal  to  each 
other.  This  can  easily  be  determined.  We  note  the  origi- 
nal position  of  the  phosphorescent  spot  on  the  glass  before 
placing  the  apparatus  in  the  magnetic  field.  When  the 
apparatus  is  placed  in  the  magnetic  field  the  beam  of 
cathode  particles  is  deflected,  and  the  bright  spot  on  the 
glass  changes  its  position.  The  electrostatic  force,  acting 
counter  to  the  magnetic,  causes  the  beam  to  occupy  more 
nearly  its  original  position.  When  these  two  opposing 
forces  are  equal  the  phosphorescent  spot  occupies  its  origi- 
nal position.  Thus  we  have  an  easy  and  efficient  means 
of  determining  when  these  two  opposite  forces  are  equal. 
When  they  are,  fev  =  se. 

Knowing  now  the  value  of  v,  and  having  previously 
determined,  as  we  have  seen,  the  ratio  of  e  to  vm,  we  have 

the  value  of  —  which  is  the  quantity  desired. 


THE  ELECTRICAL  CONDUCTIVITY  OF  GASES 


e 

THE  RATIO  —  CONSTANT  FOR  DIFFERENT  GASES 

m 
Using  a  somewhat  different  method,  J.  J.  Thomson  found 

at  first  that  the  ratio  —  was  a  constant,  whether  the  gas  in 
m 

the  tube  was  air,  carbon  dioxide,  methyl  iodide,  or  hydrogen. 
This  is  a  most  important  fact,  as  we  shall  see. 

Thomson  and  his  coworkers  then  changed  the  nature 
of  the  metal  of  which  the  cathode  was  made,  using  plati- 
num, aluminium,  silver,  copper,  tin,  zinc,  lead,  and  iron, 
to  see  whether  the  nature  of  the  metal  from  which  the 
cathode  discharge  takes  place  has  any  effect  on  the  value  of 

the  ratio  — .     They  found  the  same  value  for  — ,  regardless 
m  m 

of  the  nature  of  the  metal  of  which  the  cathode  was  made. 

Thomson  found  that  the  value  of  —  was  equal  to  about 

m 

iXio.7 

THE      RATIO      —  VARIES     FOR     THE     DIFFERENT     IONS     OF 

m 

ELECTROLYTES 

It  will  be  seen  that  the  value  of  —  for  the  ions  of  elec- 

m 

trolytes  varies  with  every  kind  of  ion.  This  is  necessarily 
the  case,  since  the  charge  carried  is  the  same  for  all  univa- 
lent  ions  (and  this  quantity  multiplied  by  the  valency  for 
all  polyvalent  ions,  as  is  seen  from  Faraday's  law),  and  the 
mass  varies  with  every  cation  and  every  anion.  Taking  the 

o 

ion  characteristic  of  acids,  hydrogen,  the  value  of  —    for 

m 

the  hydrogen  ion  is  i  X  io4. 

It  is  therefore  obvious  that  the  value  of  —  for  the  cathode 


8          THE  ELECTRICAL  NATURE  OF  MATTER 

particle  is  one  thousand  times  as  great  as  the  corresponding 
value  for  the  hydrogen  ion  produced  when  any  acid  is  dis- 
solved in  a  dissociating  solvent. 

Knowing  the  values  of  —  in  the  two  cases  does  not  tell 
m 

us  anything  about  the  relative  masses  of  the  hydrogen  ion 
in  solution,  and  the  particle  in  the  cathode  discharge;  since 
the  charges  carried  in  the  two  cases  might  be  the  same  or 
might  be  very  different.  Before  answering  this  question 
we  must  know  the  relative  charges  carried  by  the  ion  in 
electrolysis,  and  by  the  cathode  particle. 

e 

THE  VALUE  OF  —  FOR  GASEOUS  IONS  PRODUCED  BY 

m 

DIFFERENT  MEANS 

Before  taking  up  the  beautiful  method  for  determining 
the  value  of  the  charge  carried  by  the  cathode  particle,  we 

a 

shall  ask  and  answer  the  question  whether  the  value  of  — 

m 

for  gaseous  ions  is  the  same,  regardless  of  the  means 
by  which  the  gaseous  ions  are  produced,  or  whether  it 
varies  with  the  means  employed  to  produce  the  ions  in  the 
gas. 

The  answer  to  this  question  is  unmistakably  given  by 
the  results  that  have  been  obtained.  The  Lenard  rays 
are  nothing  but  cathode  rays  that  have  left  the  so-called 
vacuum  tube  by  passing  through  a  thin  sheet  of  aluminium. 

M 

The  value  of  -'-  for  the  particles  in  these  rays  has  been 
m 


found  to  be  about  4X10°. 

The  value   of  —  for  the 
m 

tact  with  incandescent  metals  is  about  8.5  Xio8. 


The  value   of  —  for  the  gaseous  ions  produced  in  con- 


THE  ELECTRICAL  CONDUCTIVITY  OF  GASES 


The  value  of  —  for  the  negative  ion  given  off  from  radio- 
m 

active  substances  is  about   iXio7. 

It  is  obvious  that  the  above  values  all  refer  to  the  nega- 
tive, gaseous  ion.     We  see  from  the  results  that  the  value 

of  —  for  this  ion  is  practically  constant,  regardless  of  the 
m 

means  by  which  it  is  produced,  and  regardless  of  the  nature 
of  the  gas  from  which  it  is  produced. 

o 

As  to  the  value  of  —  for  the  positive  ion  of  gases,  we 

shall  have  something  to  say  in  the  next  chapter,  and  shall 
also  discuss  the  nature  of  this  ion. 


CHAPTER  II 

THE  DETERMINATION  OF  THE   MASS  OF  THE  NEGATIVE 
ION  IN  GASES 

WORK  OF  J.  J.  THOMSON 

THE  determination  of  the  charge  carried  by  the  negative 
ion  is  of  the  very  greatest  importance.  We  have  already 

M 

considered  the  method  for  determining  the  ratio  —  for  the 

m 

negative  ion.  If  we  can  now  determine  e,  the  charge  carried 
by  this  ion,  we  would  know  m,  the  mass  of  the  negative  ion. 

One  of  the  most  ingenious  experiments  in  modern  physics 
has  been  devised  by  J.  J.  Thomson  for  solving  this  problem. 
The  experiment  is  based  on  an  observation  made  by  C.  T. 
R.  Wilson,1  that  gaseous  ions,  both  positive  and  negative, 
can  act  as  nuclei  for  the  condensation  of  water- vapor,  even 
if  there  are  no  dust  particles  present  in  the  gas.  If  a  given 
volume  of  a  gas  containing  ions  is  allowed  to  expand,  it 
cools  itself,  and  a  part  of  the  water-vapor  will  condense 
around  the  ions,  producing  a  fog  or  cloud  in  the  apparatus 
containing  the  gas. 

That  the  water-vapor  actually  condenses  around  the 
ions  was  proved  conclusively  by  J.  J.  Thomson,  by  the 
following  very  simple  experiment.  Two  parallel  metal 
plates  were  placed  a  few  centimetres  apart  in  the  vessel 
containing  the  gas  which  had  been  freed  fiom  dust.  These 

'Phil.  Trans.,  A.,  265  (1897). 
10 


DETERMINATION  CF  THE  MASS  OF  THE  NEGATIVE  ION       1 1 

plates  were  connected  with  the  terminals  of  a  battery,  by 
which  they  could  be  charged  to  a  relatively  high  difference 
in  potential.  Ions  were  produced  in  the  gas  by  passing 
Rontgen  rays  through  it. 

If  the  gas  was  expanded  before  the  plates  were  connected 
with  the  battery,  condensation  of  the  vapor  took  place; 
just  as  we  should  expect  if  the  ions  acted  as  nuclei  around 
which  the  water-vapor  would  condense.  If  the  plates  are 
now  connected  with  the  battery  and  charged,  the  strong 
electrical  field  would  remove  the  ions  from  the  gas,  and  if 
the  gas  were  then  subjected  to  expansion  we  would  not  ex- 
pect any  appreciable  condensation  to  take  place,  and  such 
is  the  fact.  Thomson  says  that  under  these  conditions  the 
condensation  is  scarcely  greater  than  in  unionized  air. 

This  experiment  shows  conclusively  that  it  is  the  ions 
that  serve  as  the  centres  of  condensation  of  the  water- 
vapor  —  a  drop  of  water  condensing  around  every  ion  if 
the  ions  are  not  too  numerous.  If  we  knew  the  number 
of  droplets  of  water  in  a  given  volume  of  the  gas,  we  would 
know  the  number  of  ions  in  that  volume.  It  is,  however, 
obviously  impossible  to  determine  the  number  of  water 
particles  in  a  volume  of  gas  by  any  direct  method.  Thom- 
son l  solved  this  part  of  the  problem  by  using  an  equation 
deduced  by  Stokes,  connecting  the  rate  at  which  the  par- 
ticles fall  with  their  size.  If  we  represent  by  v  the  velocity 
with  which  the  particles  fall,  by  g  the  acceleration  of  gravity, 
by  c  the  viscosity  coefficient  of  the  gas,  and  by  r  the  radius 

of  the  drop, 

_2grz 

v  — 

9  ^ 
By  observing  the  rate  at  which  the  cloud  settles  we  arrive 

'Phil.  Mag.,  46,  528(1856). 


12         THE  ELECTRICAL  NATURE  OF  MATTER 

at  the  value  of  v.  Knowing  v  we  determine  at  once  the 
value  of  r,  the  radius  of  the  drop.  Knowing  the  radius  of 
the  drop  we  know  its  volume. 

If  we  represent  the  mass  of  the  water  deposited  by  each 
cubic  centimetre  of  the  gas  by  M,  the  number  of  drops  in  a 
cubic  centimetre  n  is  given  by  the  following  equation: 


, 

4777 


The  mass  of  water  deposited  from  each  cubic  centimetre 
of  the  gas,  M,  must  be  determined  indirectly.  Thomson 
made  use  of  the  heat  that  is  liberated  when  the  water-  vapor 
condenses  around  the  gaseous  ions.  Knowing  M  and  r, 
we  have  all  the  data  necessary  for  calculating  n,  the  num- 
ber of  ions  in  a  cubic  centimetre  of  the  gas,  which  is  equal 
to  the  number  of  droplets  in  the  same  volume. 

We  now  know  the  number  of  ions  in  a  given  volume  of 
the  gas.  It  still  remains  to  determine  the  charge  carried 
by  a  single  ion. 

If  we  knew  the  total  quantity  of  electricity  carried  by  the 
known  number  of  ions,  we  would  know  the  amount  carried 
by  one  ion.  Let  v  be  the  mean  velocities  of  both  positive 
and  negative  ions  when  subjected  to  unit  electrical  force. 
We  must  measure  the  current  carried  by  these  ions  across 
unit  area,  under  an  electric  force  F,  in  order  to  determine 
the  charge  carried  by  a  single  ion.  If  we  represent  the 
charge  carried  by  a  single  ion  as  formerly  by  e,  we  have: 
Fvne  =  current  through  unit  area  perpendicular  to  the 
current.  Measuring  the  current  that  passes  through  the 
gas,  we  know  all  of  the  above  quantities  except  e,  which  is 
calculated  at  once. 

In  performing  the  condensation  experiment  it  is  neces- 


DETERMINATION  OF  THE  MASS  OF  THE  NEGATIVE  ION     13 

sary,  as  Thomson  points  out,  to  work  with  gases  that  con- 
tain only  a  comparatively  small  number  of  ions.  When  the 
conducting  gas  contains  a  large  number  of  ions  some  of 
these  are  not  carried  down  by  the  condensed  water-vapor, 
as  is  shown  by  the  fact  that  under  these  conditions  a  second 
expansion  of  the  gas,  which  is  no  longer  subjected  to  the 
ionizing  agent,  will  produce  still  further  condensation, 
demonstrating  that  it  still  contains  ions  that  were  not  carried 
down  by  the  first  expansion. 

The  condition  that  the  gas  shall  contain  only  a  few  ions 
is  easily  secured,  especially  when  the  gas  is  ionized  by 
Rontgen  rays.  Either  a  weak  stream  of  the  rays  is  allowed 
to  pass  directly  through  the  gas,  or  the  intensity  of  the  rays 
is  diminished  by  inserting  thin  sheets  of  certain  metals, 
such  as  aluminium,  in  their  path. 

The  earlier  experiments  showed  that  the  values  of  e  for 
air  ionized  by  Rontgen  rays,  and  for  hydrogen  gas  ionized 
by  Rontgen  rays,  are  equal  to  within  the  limit  of  experi- 
mental error,  which  proves  that  the  gaseous  ion  carries  the 
same  charge  whatever  the  gas  from  which  it  was  produced. 
Ir*is  of  the  order  of  magnitude  4Xicr10. 

COMPARISON    OF    THE    CHARGE    ON    A    GASEOUS    ION    WITH 
THAT  ON  A  UNIVALENT  ION  OF  AN  ELECTROLYTE 

Having  determined  the  magnitude  of  the  charge  on  a 
gaseous  ion,  we  shall  next  determine  the  magnitude  of  the 
charge  carried  by  a  univalent  ion  of  an  electrolyte  —  say 
the  hydrogen  ion. 

We  know  that  the  number  of  molecules  in  a  cubic  centi- 
metre of  a  gas,  at  a  pressure  of  760  millimetres  of  mercury 
and  at  zero  degrees,  is  between  2Xio19  and  iXio20.  We 
know  the  amount  of  electricity  required  to  liberate  this 


14         THE  ELECTRICAL  NATURE  OF  MATTER 

amount  of  hydrogen  gas.  From  these  data  we  calculate 
that  the  charge  carried  by  the  hydrogen  ion  in  solution  is 
somewhere  between  iXio~10  and  6Xio~10. 

We  thus  see  that  the  charge  carried  by  the  gaseous  ion  is 
the  same  as  that  carried  by  the  hydrogen  ion  in  electrolysis. 

This  conclusion  is  based  upon  a  large  amount  of  work 

with  the  ions  produced  from  various  gases  and  by  various 

^ 
ionizing  agents.     We  have  already  seen  that  the  value  o)  - 

for  all  of  these  gaseous  ions  is  the  same,  no  matter  what  the 
nature  of  the  gas  from  which  they  were  produced,  and  no 
matter  what  the  nature  of  the  ionizing  agent.  It  has  further 
been  shown  that  all  of  these  gaseous  ions  carry  the  same 
charge,  and  that  this  is  the  same  charge  as  that  carried  by 
the  hydrogen  ion  in  aqueous  solution. 

We  have  now  all  the  data  necessary  for  calculating  the 
relative  masses  of  the  gaseous  ion,  and  the  hydrogen  ion  in 
solution. 

The  value  of  —  for  the  hydrogen  ion  in  solution  is  io4. 
m 

The  value  of  —  for  the  gaseous  ion  is  io7.    The  values  of  e 
m 

in  the  two  cases  are  the  same.  Therefore,  the  value  of  m 
for  the  gaseous  ion  is  about  one-thousandth  the  value  of  m 
for  the  hydrogen  ion  in  solution. 

More  accurate  determinations  show  that  the  relation 
between  the  masses  of  the  gaseous  ion  and  the  hydrogen 
ion  in  solution  is  as  i  to  about  1765. 

It  is  difficult  to  overestimate  the  importance  of  this  con- 
clusion. In  the  first  place,  it  is  a  matter  of  the  very  highest 
importance  to  establish  the  fact  that  the  mass  of  the  gaseous 
negative  ion  is  always  the  same,  no  matter  what  the  nature 
of  the  gas  from  which  this  ion  is  split  off,  and  no  matter 


DETERMINATION  OF  THE  MASS  OF  THE  NEGATIVE  ION     1 5 

what  the  nature  of  the  ionizing  agent.  This  has  been 
shown  to  be  true  whether  the  gas  is  elementary  or  compound. 
This  shows  that  a  common  constituent  can  be  split  off  from 
all  gases  no  matter  how  widely  they  may  differ  chemically, 
and  what  is  perhaps  even  more  important  is  that  the  mass 
of  this  negative  ion  which  can  be  split  off  from  any  gas  is 
much  less  than  the  mass  of  the  lightest  so-called  element  known 
to  the  chemist.  The  gaseous  negative  ion  is,  then,  a  com- 
mon constitutent  of  all  matter,  and  is  much  smaller  than 
the  smallest  atom  known  to  the  chemist,  having  a  mass 
which  is  only  about  i^Ys  °f  tnat  °f  tne  hydrogen  ion  in 
solution,  which,  as  we  shall  see,  has  practically,  but  not 
exactly,  the  same  mass  as  the  hydrogen  atom. 

This  unit  of  matter,  so  much  smaller  than  the  atom, 
and  which  is  apparently  common  to  all  atoms,  carrying  a 
unit,  negative  electrical  charge  or  that  charge  carried  by 
the  chlorine  ion  in  solution,  Thomson  called  a  corpuscle. 

THE  RATIO  OF  THE  CHARGE  TO  THE  MASS  FOR  THE 
POSITIVE  ION 

Before  leaving  this  part  of  our  subject  a  few  words  should 

be  added  in  relation  to  the  value  of  the  ratio  —  for  the 

m 

positive  ion.  These  positive  ions  exist  in  the  so-called 
canal  rays,  discovered  by  Goldstein.  They  are  also 
known  as  anode  rays.  Just  as  the  cathode  rays  move 
from  the  cathode  towards  the  anode,  so  there  is  a  corre- 
sponding movement  of  matter  towards  the  cathode.  This 
can  be  detected  by  perforating  the  cathode  with  a  number 
of  holes,  through  which  the  canal  rays  pass,  and  pro- 
duce a  phosphorescence  where  they  fall  on  the  walls  of  the 
glass  tube  behind  the  cathode. 

Wien  used  a  perforated  cathode  of  iron,  and  determined 


1  6         THE  ELECTRICAL  NATURE  OF  MATTER 


the  value  of  --  for  the  rays  which  passed  through  his 
cathode.  He  used  the  method  already  described  for  deter- 
mining the  value  of  —  for  the  cathode  particles.  He  de- 

flected the  rays  by  means  of  a  strong  magnetic  field,  and 
then  in  the  opposite  direction  by  means  of  an  electrostatic 
field.  A  strong  magnetic  field  is  necessary  to  produce  an 
appreciable  deflection  of  the  canal  rays,  and  this  renders 
the  result  less  accurate.  He  obtained  the  following  average 
result: 


He  also  found  that  these  positively  charged  particles  move 
with  much  smaller  velocity  than  the  negatively  charged 

particles. 

& 

If  we  compare  the  value  of  —  for  the  negatively  charged 

m 

particle  with  that  for  the  positively  charged  iron  particle, 
we  shall  see  that  the  value  for  the  negatively  charged 
particle  is  about  3.3X10*  times  the  value  for  the  positively 
charged  particle. 

Since  the  electricity  carried  by  the  positively  charged  par- 
ticle is  the  same  in  quantity  as  that  carried  by  the  nega- 
tively charged  particle,  it  follows  that  the  mass  of  the  positive 
particle  is  of  the  same  order  of  magnitude  as  that  of  the 
corresponding  ion  in  solution. 

We  can,  then,  conclude  that  while  the  mass  of  the  nega- 
tively charged  particle  in  a  gas  is  constant,  independent  of 
the  nature  of  the  gas,  and  very  small  as  compared  even 
with  the  mass  of  the  lightest  atom  or  ion  in  solution,  the 
mass  oj  the  positively  charged  particle  is  of  the  same  order 
o)  magnitude  as  the  corresponding  atom  or  ion  in  solution  in 
a  dissociating  solvent.  The  mass  of  the  positively  charged 


DETERMINATION   OF   THE   MASS   OF   THE  NEGATIVE   ION    17 

particle  is  not  constant  for  different  gases,  but,  as  we  should 
expect  if  the  positive  ion  is  a  charged  atom,  varies  with  the 
nature  of  the  gas  in  question. 

The  recent  work  of  Thomson1  on  the  ratio  of  —  for 

m 

the  positively  charged  particles  gave  the  following  values. 
Wien  obtained  values  as  high  as  io4  for  the  more  readily 
deflected  positive  particles.  Thomson  found  in  air  as  in 
hydrogen  i.2Xio4,  while  another  set  of  particles  in  hy- 
drogen gave  the  value  2.gXio3.  In  argon  he  found  the 
value  io4. 
In  gases  at  low  pressures  essentially  the  same  values 

o    . 

were  found  for  the  ratio  — .    When  the  pressure  was  low 

m 

or  the  dilution  of  the  gas  great,  there  were  generally 
streams  of  two  kinds  of  carriers,  one  having  the  value  of 

—  =  io4,  and  the  other  the  value  about  5  X  io3. 
m 

It  should  be  noted  that  these  are  the  approximate  values 
for  the  charged  atom  and  the  charged  molecule  of  hydro- 
gen. An  elaborate  study  of  positive  rays  has  recently  been 
made  by  Thomson.2  To  give  an  idea  of  the  complexity 
of  the  phenomena  in  a  discharge  tube,  Thomson  calls 
attention  to  the  fact  that  Goldstein,3  who  discovered  the 
"Canal  Rays,"  discovered  five  kinds  of  rays  besides  the 
cathode  rays.  This  led  Thomson  to  undertake  the  above- 
mentioned  investigation. 

An  elaborate  investigation  of  the  positive  rays  has  been 
made  by  Stark,  but  the  scope  of  this  book  will  not  permit 
of  a  discussion  of  this  work. 

This  beautiful  work  of  Thomson,  on  the  conduction  of 

^hil.  Mag.,  13,  561  (1907);  14,  359  (1907). 

zlbid. 

3  Ibid.,  16,  657  (1908). 


1 8        THE  ELECTRICAL  NATURE  OF  MATTER 

electricity  through  gases,  makes  it  more  than  probable  that 
a  small  particle  which  he  calls  the  corpuscle  is  split  off  from 
the  atoms  of  all  gases,  carries  the  negative  charge,  and  is 
the  same  unit,  no  matter  what  the  nature  of  the  atom  from 
which  it  separates. 

The  remainder  of  the  atom  from  which  the  corpuscle  has 
separated  carries  the  positive  charge,  and  is  the  positively 
charged  ion  in  the  gas.  The  nature  of  this  positive  ion  is 
different  for  every  gas,  being  simply  the  atom  minus  the  con- 
stituent common  to  all  atoms,  which  is  the  corpuscle. 

It  would  be  a  tremendous  step  forward  towards  the  solu- 
tion of  one  of  the  greatest  problems  with  which  men  of 
science  have  had  to  deal  —  the  ultimate  nature  of  matter  — 
had  Thomson  gone  no  farther  than  what  has  been  above 
developed.  This  is,  however,  but  the  beginning.  Thom- 
son has  studied  the  nature  of  the  corpuscle  itself,  and  the 
result  of  this  part  of  his  investigation  is  certainly  one  of 
the  most  fascinating,  and  probably  one  of  the  most  valuable 
contributions  to  modern  science. 


CHAPTER  III 

NATURE  or  THE  CORPUSCLE  —  THE  ELECTRICAL  THEORY 
OF  MATTER 

THE  conception  of  the  corpuscle  as  originally  advanced  is 
that  it  is  a  small  piece  of  matter  having  a  mass  about  ifW 
of  that  of  the  hydrogen  atom,  and  carrying  a  unit  negative 
charge  of  electricity,  which  is  exactly  the  same  as  that  car- 
ried by  any  univalent  anion,  such  as  the  chlorine  ion  in  solu- 
tion. The  corpuscle  is  thus  both  material  and  electrical 
in  its  nature. 

We  shall  now  take  up  Thomson's  study  of  the  corpuscle 
itself,  and  see  how  the  original  conception  has  been  modi- 
fied, and  the  reasons  for  the  view  that  we  hold  at  present. 

Let  us  first  ask  what  reason  have  we  for  supposing  that 
the  corpuscle  contains  any  matter  at  all?  How  do  we 
know  that  it  is  anything  but  electricity  ?  The  answer  would 
be  that  the  corpuscle  has  both  mass  and  inertia,  and,  there- 
fore, must  contain  matter,  since  matter  only  has  these  proper- 
ties. We  shall  now  see  whether  this  line  of  reasoning  is 
valid. 

WORK  OF  THOMSON  AND  KAUFMANN 

In  a  paper  published  a  number  of  years  ago,  J.  J. 
Thomson  at  least  raised  the  question  as  to  whether  inertia 
itself  is  not  of  electrical  origin.  The  mass  of  a  charged 
sphere  would,  in  this  case,  be  greater  than  that  of  the  same 
sphere  when  uncharged. 

19 


20         THE  ELECTRICAL  NATURE  OF  MATTER 

Thomson  showed  that  the  particle  must  move  very  rapidly 
in  order  to  have  appreciable  changes  in  its  mass.  Indeed, 
it  must  move  with  a  velocity  which  is  comparable  with  that 
of  light,  in  order  to  produce  measurable  changes  in  its  mass. 

While  the  ordinary  cathode  rays  move  with  a  velocity 
that  is  only  about  3X10°  centimetres  per  second,  the 
particles  shot  off  from  radium  have  a  velocity  as  high  as 
2.8Xio10,  which  is  nearly  that  of  light  itself  =  3Xio10. 
If  the  velocity  with  which  the  charge  moves  has  any  effect 
on  its  apparent  mass,  we  should  expect  that  the  mass  of 
these  rapidly  moving  particles  would  be  greater  than  that 
of  the  same  particles  when  moving  less  rapidly.  This 

question  has  been  answered  by  the  experiments  of  Kauf- 

(> 

mann.1    He   determined    the    value  of  —  for  these  more 

m 

rapidly  moving  particles,  by  means  of  the  method  already 
described,  using  the  magnetic  and  electrical  deflections. 
He  found  values  as  low  as  0.63  Xio7  for  the  most  rapidly 
moving  particles.  Since  e  is  constant,  the  charge  being 
the  same  independent  of  the  velocity,  it  follows  that  the 
mass  of  the  rapidly  moving,  charged  particle  is  greater  than 
that  of  the  more  slowly  moving,  charged  particle. 

Kaufmann's  experiments  went  farther.  By  means  of 
the  electrical  and  magnetic  deflections,  he  determined  the 

p 

values  of  —  for  the  f$  particles  shot  off  from  radium  with 

different  velocities.  We  shall  learn  that  these  are  essen- 
tially cathode  ray  particles.  He  obtained  the  following 

results.    The    velocities  v   are   divided   by   io10,   and   the 
0 

values  of  —  by  io7,  for  convenience.     The  figures  give  us 
m 

the  relative  values,  which  are  all  that  we  desire  at  present: 

1  Phys.  Zeit.,  4,  54  (1902). 


NATURE   OF   THE   CORPUSCLE  21 

e 
V 

m 

2.36  1.31 

2.48  1.17 

2-59  o-975 

2.72  0.77 

2.83  0.63 

It  is  obvious  from  the  above  data  that  as  the  velocity  of 
the  charged  particle  increases,  the  value  of  --  decreases. 

Since  the  value  of  the  charge,  e,  remains  constant,  inde- 
pendent of  the  velocity,  it  follows  that  the  mass  m  becomes 
greater  and  greater  as  the  velocity  of  the  charged  particle 
becomes  greater. 

The  experiments  of  Kaufmann  show  conclusively  that 
the  mass  of  a  charged  particle  changes  with  the  velocity  of 
the  particle,  increasing  as  the  velocity  increases.  In  a 
word,  a  part  of  the  mass  0}  the  particle,  at  least,  is  o]  electrical 
origin. 

This  would  naturally  raise  the  question,  what  part  of 
the  mass  is  electrical?  Is  it  possible  that  all  mass  is  elec- 
trical? Thomson  has  thrown  light  on  this  question  in  the 
following  manner.  When  the  corpuscle  moves  slowly  the 
mass,  as  we  have  seen,  does  not  depend  on  the  velocity, 
and  does  not,  therefore,  change  with  the  velocity.  When, 
on  the  other  hand,  the  velocity  of  the  corpuscle  approaches 
the  velocity  of  light,  the  mass  varies  with  the  velocity,  as 
is  shown  by  the  results  of  Kaufmann.  Assuming  that  the 
entire  mass  of  the  corpuscle  is  of  electrical  origin,  Thom- 
son has  calculated  the  variation  of  the  masses  of  the 
particles  writh  the  velocity. 

The    agreement    between    the    calculated    and    observed 


22         THE  ELECTRICAL  NATURE  OF  MATTER 

values  is  surprisingly  good.  This  is  a  strong  argument  in 
favor  of  the  correctness  of  the  assumption  on  which  the 
calculation  is  based. 

If  the  whole  mass  of  the  corpuscle  is  electrical,  why  assume 
that  the  corpuscle  contains  any  so-called  matter  at  all? 
All  of  the  properties  of  the  corpuscle,  including  the  two 
properties  that  we  have  been  accustomed  to  associate  with 
matter,  inertia  and  mass,  are  accounted  for  by  the  electrical 
charge  of  the  corpuscle.  Since  we  know  things  only  by 
their  properties,  and  since  all  of  the  properties  of  the  cor- 
puscle are  accounted  for  by  the  electrical  charge  associated 
with  it,  why  assume  that  the  corpuscle  contains  anything 
but  the  electrical  charge?  It  is  obvious  that  there  is  no 
reason  for  doing  so. 

The  corpuscle  is,  then,  nothing  but  a  disembodied  electrical 
charge,  containing  nothing  material,  as  we  have  been  accus- 
tomed to  use  that  term.  It  is  electricity,  and  nothing  but 
electricity.  With  this  new  conception  a  new  term  was 
introduced,  and,  now,  instead  of  speaking  of  the  corpuscle 
we  speak  of  the  electron.  /  The  electron  is,  then,  a  disem- 
bodied electrical  charge,  containing  no  matter,  and  is  the 
term  which  we  shall  hereafter  use  for  this  ultimate  unit,  of 
which  we  shall  learn  that  all  so-called  matter  is  probably 
composed. 

If  the  electron  contains  nothing  that  corresponds  to  our 
ordinary  conception  of  matter,  and  since  the  same  electron 
can  be  split  off  from  the  atoms  or  molecules  of  all  sub- 
stances, the  question  naturally  arises,  is  not  all  so-called 
matter  made  up  of  these  electrical  charges  or  electrons? 
Is  not  all  matter  of  an  electrical  nature  ?  There  is  a  large 
amount  of  evidence,  part  of  which  has  already  been  given, 
which  answers  this  question  in  the  affirmative.  Indeed, 
this  conclusion  is  accepted,  at  least  tentatively,  by  a  large 


NATURE   OF   THE   CORPUSCLE  23 

number  of  the  leading  physicists    and    physical    chemists 
the  world  over. 

THE  ELECTRON  THE  ULTIMATE  UNIT  OF  MATTER 

According  to  the  above  theory  the  electron  is  the  ultimate 
unit  of  all  matter.  The  atoms  are  made  up  of  electrons 
or  disembodied  electrical  charges,  in  rapid  motion;  the  atom 
oj  one  elementary  substance  differing  jrom  the  atom  of  another 
elementary  substance  only  in  the  number  and  arrangement 
of  electrons  contained  in  it.  Thus  we  have  at  last  the  ulti- 
mate unit  of  matter,  of  which  all  forms  of  matter  are  com- 
posed; and  the  remarkable  feature  is,  that  this  ultimate 
unit  of  which  all  matter  is  composed  is  not  matter  at  all,  as 
we  ordinarily  understand  that  term,  but  electricity. 

This  recalls  a  paper  published  a  number  of  years  ago 
by  Ostwald,1  on  " The  Overthrow  of  Scientific  Materialism," 
which  made  an  impression  at  the  time  that  it  appeared,  or 
rather  a  number  of  impressions.  The  arguments  and  con- 
clusions in  this  paper  were  accepted  by  some  without  ques- 
tion, and  were  severely  criticised  by  others,  especially  by 
the  mathematical  physicists  of  Germany.  Whatever  our 
opinion  of  the  paper  as  a  whole,  there  is  one  point  at  least 
brought  out  so  clearly  that  there  can  scarcely  be  any  ques- 
tion about  it,  and  that  is,  that  matter  is  a  pure  hypothesis. 

What  we  know  in  the  universe,  and  all  that  we  know,  is 
changes  in  energy.  In  order  to  have  something  to  which 
we  can  mentally  attach  the  energy,  we  have  created,  in  our 
imagination,  matter. 

Matter,  then,  is  a  pure  hypothesis,  and  energy  is  the  only 
reality,  We  are  accustomed  to  take  exactly  the  opposite 
view,  and  regard  matter  as  the  reality  and  energy  as  hy- 
pothetical. If  Ostwald  accomplished  nothing  else  by  the 

1  Zeit.  phys.  Chem.,  18,  305  (1895). 


24        THE  ELECTRICAL  NATURE  OF  MATTER 

paper  in  question  than  the  mere  calling  attention  to  the 
hypothetical  nature  of  matter,  he  made  an  important  con- 
tribution to  science. 

It  should  also  be  noted  that  for  a  long  time  Ostwald  has 
insisted  not  only  that  matter  is  a  pure  hypothesis,  but  there 
is  not  the  least  evidence  for  its  existence,  as  we  ordinarily 
understand  the  term.  It  is  interesting  to  note  that  Thom- 
son has  reached  the  same  conclusion,  as  the  result  of  one 
of  the  most  brilliant  series  of  experiments  that  has  ever 
been  carried  out  in  any  branch  of  experimental  science. 
We  thus  have  a  direct  experimental  verification  of  a  conclu- 
sion, the  importance  of  which  it  is  difficult  to  overestimate. 

EARLIER  ATTEMPTS  TO  UNIFY  MATTER 

Perhaps  the  most  important  bearing  of  the  electron  is 
that  it  furnishes  us  with  the  ultimate  basis  of  all  matter. 
The  importance  of  securing  such  an  ultimate  unit  is  shown 
by  the  number  of  attempts  that  have  been  made  in  this 
direction.  One  of  the  first  noteworthy  efforts  we  owe  to 
the  chemist  Prout.  After  fairly  accurate  determinations  of 
the  atomic  weights  of  a  number  of  the  more  common  ele- 
ments had  been  made,  it  appeared  that  when  these  values 
were  expressed  in  terms  of  the  atomic  weight  of  hydrogen 
as  unity,  they  were  all  nearly  whole  numbers.  Indeed,  the 
deviations  at  first  discovered  were  hardly  greater  than  the 
experimental  errors. 

This  led  Prout,  as  early  as  1815,  to  propose  the  theory 
that  hydrogen  is  the  ultimate  element  of  which  all  other 
elementary  substances  are  made.  The  atoms  of  all  other 
elements  are  simply  condensations  of  hydrogen  atoms,  the 
number  of  hydrogen  atoms  contained  in  an  atom  of  any 
element  being  expressed  by  the  atomic  weight  of  the  element 
in  terms  of  hydrogen  as  unity. 


NATURE   OF   THE   CORPUSCLE  25 

This  hypothesis  of  Prout  accounted  for  all  the  facts  that 
were  known  at  the  time  when  it  was  proposed,  and  it  is  a 
praiseworthy  attempt  to  solve  the  problem  of  the  relation 
between  the  various  chemical  elements. 

As  experimental  methods  became  more  refined,  and 
atomic  weights  more  accurately  determined,  it  gradually 
became  obvious  that  the  atomic  weights  of  even  some  of 
the  more  common  elements  are  not  whole  numbers  in  terms 
of  hydrogen  as  one,  but  differ  very  appreciably  from  whole 
numbers.  Indeed,  the  atomic  weights  of  some  elements 
fall  almost  half-way  between  whole  numbers.  This  was, 
of  course,  a  deviation  too  large  to  be  accounted  for  on  the 
basis  of  experimental  error,  and  was,  therefore,  the  death 
blow  to  the  hypothesis  of  Prout  as  it  was  originally  proposed 
by  its  author. 

Subsequent  suggestions  by  Marignac  and  others  to  make 
the  half-atom  of  hydrogen,  or  even  the  quarter-atom,  the 
basis  of  all  matter,  did  not  increase  scientific  respect  for 
the  hypothesis  of  Prout.  Having  once  begun  to  divide  the 
hydrogen  atom  the  process  could  be  continued  indefinitely, 
and  thus  the  theory  could  be,  and  was  for  a  time,  brought 
into  disrepute. 

It  must  not,  however,  be  forgotten  that  if  to-day  we  take 
those  chemical  elements  whose  atomic  weights  are  most 
accurately  determined,  and  calculate  the  atomic  weights 
on  the  basis  of  oxgyen  =  16,  which  is  the  system  now  in 
general  use,  we  shall  find  that  a  very  large  proportion  of  the 
atomic  weights  are  so  close  to  whole  numbers  that  the 
deviations  can  be  accounted  for  on  the  basis  of  probable 
experimental  errors. 

Such  an  examination  was  recently  made  by  Strutt,1  who 
pointed  out  that  the  number  of  elements  whose  atomic 

1  Phil.  Mag.,  i,  311  (1901). 


26        THE  ELECTRICAL  NATURE  OF  MATTER 

weights  are  whole  numbers  is  many  times  too  large  to  be 
accounted  for  on  the  basis  of  chance. 

Taking  all  of  the  facts  into  account,  we  recognize,  of 
course,  that  the  hypothesis  of  Prout,  either  as  originally 
proposed,  or  as  subsequently  modified,  is  not  rigidly  true; 
but  we  still  feel  intuitively  that  there  is  something  in  it. 
The  coincidences  are  far  too  numerous  to  be  attributed  to 
mere  chance. 

OTHER  RELATIONS  BETWEEN  THE  ELEMENTS 

A  number  of  other  attempts  have  been  made  to  point 
out  relations  between  the  atoms  of  the  different  chemical 
elements,  with  the  hope  of  finding  something  in  common 
between  them.  Dobereiner  early  noticed  that  of  three 
closely  related  chemical  elements,  the  atomic  weight  of  the 
second  heaviest  element  is  almost  exactly  the  mean  of  the 
atomic  weights  of  the  lightest  and  heaviest  elements  of 
the  group  of  three.  A  few  examples  will  make  this  clear. 
Take  the  three  elements,  calcium,  strontium  and  barium. 
The  atomic  weight  of  calcium  is  40.07;  the  atomic  weight 
of  barium  is  137.37.  The  mean  of  these  two  values  is 
88.72,  and  the  atomic  weight  of  strontium  is  87.63.  To 
take  an  example  from  the  negative  elements,  the  above 
being  taken  from  the  positive,  let  us  choose  sulphur,  se- 
lenium and  tellurium.  The  atomic  weight  of  sulphur  is 
32.07;  that  of  tellurium  127.5.  The  mean  of  these  two 
values  is  79.78,  while  the  atomic  weight  of  selenium  is  79.2 

Relations  such  as  these  are,  of  course,  purely  empirical, 
and  their  meaning  is  entirely  unknown,  yet  they  are,  to 
say  the  least,  suggestive. 

We  now  come  to  the  great  generalization  of  Newlands, 
Mendeleeff,  and  Lothar  Meyer,  known  as  the  Periodic 
System.  This  is  the  only  attempt  thus  far  made  to  coor- 


NATURE   OF  THE   CORPUSCLE  27 

dinate  all  of  the  chemical  elements  into  one  comprehensive 
system.  The  system  is  too  well  known  to  be  discussed  at 
any  length  in  the  present  connection.  It  is  referred  to  here 
to  call  attention  to  the  most  serious  effort  that  has  ever 
been  made  to  discover  general  relations  holding  for  all  of 
the  chemical  elements. 

It  is  well  known  that  in  the  Periodic  System  the  chemical 
elements  are  arranged  in  the  order  of  their  increasing  atomic 
weights.  It  is  not  only  found  that  the  chemical  and  physi- 
cal properties  of  the  elements  are  a  function  of  their  atomic 
weights,  but  a  periodic  function  of  the  atomic  weights.  If 
we  arrange  the  elements  according  to  the  above  principle, 
in  groups  of  seven,  allowing  the  eighth  element  to  fall  under 
the  first,  it  is  well  known  that  the  elements  with  chemically 
allied  properties  will  fall  in  the  same  vertical  columns. 

It  would  lead  us  too  far  in  this  connection  to  point  out 
the  many  and  interesting  chemical  relations  brought  out 
by  the  Periodic  System,  and,  perhaps,  what  is  even  more 
important,  the  relations  between  the  atomic  weights  of  the 
elements  and  their  physical  properties.  It  is  sufficient  to 
note  here  that  such  relations  do  exist,  and  that  these  are  of 
a  general  character,  embracing  practically  all  of  the  ele- 
ments known  to  the  chemist. 

The  writer  is  in  no  sympathy  with  the  attempt  that  is 
being  made  in  certain  directions  to  belittle  and  cast  into 
the  background  the  Periodic  System.  Of  course,  every 
one  must  recognize  that  the  system  is  incomplete.  Indeed, 
it  is  not  only  far  from  being  complete,  but  leads  in  places  to 
inconsistencies.  Yet  the  Periodic  System  is  a  great  generali- 
zation, which  coordinates  an  enormous  number  of  otherwise 
disconnected  facts,  and  has  done  more  towards  placing 
inorganic  chemistry  upon  a  scientific  basis  than  all  the 
other  generalizations  together,  that  were  proposed  up  to 


28         THE  ELECTRICAL  NATURE  OF  MATTER 

1886.  Indeed,  it  was  the  philosophy  of  inorganic  chemistry 
for  a  comparatively  long  period,  and  has  far  from  lost  its 
usefulness  at  present.  As  we  shall  see,  it  again  comes  to 
the  front  in  connection  with  the  electron  theory  of  matter 
that  we  are  now  discussing. 

These  are  some  of  the  more  important  of  the  earlier 
attempts  to  discover  connections  and  relations  between 
the  different  chemical  elements.  None  of  these,  with  the 
exception  of  the  hypothesis  of  Prout,  can  be  said  to  have 
attempted  to  solve  the  problem  of  the  nature  of  the  chemical 
elements,  even  as  referred  to  some  one  known  element  as 
the  standard. 

In  the  above  very  brief  review  of  the  efforts  that  have 
been  made  to  establish  connections  between  the  various 
chemical  elements,  a  number  of  pure  speculations  by  the 
ancients  have  been  omitted.  Most  of  these  are  only  of 
historical  interest,  and  since  they  do  not  admit  of  experi- 
mental test,  are  of  little  or  no  scientific  importance. 

We  shall  turn  now  to  the  electron  theory  of  matter,  and 
study  some  of  its  applications. 


CHAPTER  IV 

THE  NATURE  OF  THE  ATOM  IN  TERMS  OF  THE  ELECTRON 

THEORY 

ACCORDING  to  the  theory  that  we  have  just  developed, 
all  atoms  of  whatsoever  kind  are  made  up  of  electrons, 
which  are  nothing  but  negative  charges  of  electricity  in 
rapid  motion.  In  accepting  this  wonderfully  simple  and 
beautiful  theory  that  the  nature  of  all  matter  is  essentially 
the  same,  we  must  not  forget  the  facts  of  chemistry  and 
physics  which  have  to  be  accounted  for.  We  must  remem- 
ber that  we  have  over  seventy  apparently  different  forms 
of  matter,  which  cannot  be  decomposed  into  anything 
simpler,  or  into  one  another,  by  any  agent  known  to  man. 
We  must  also  remember  that  these  elements  of  the  chemist 
have  each  their  definite  and  distinctive  properties,  both 
physical  and  chemical.  They  enter  into  combination  with 
one  another  in  perfectly  distinctive  ways,  and  form  com- 
pounds with  definite  and  characteristic  properties.  In  a 
word,  we  must  remember  the  almost  unlimited  facts  of 
chemical  science,  which  are  facts,  regardless  of  whatever 
conception  of  the  ultimate  nature  of  matter  we  may  hold. 

We  must  also  not  be  unmindful  of  the  great  mass  of 
facts  that  have  been  brought  to  light  as  the  result  of  the 
application  of  physical  forces  to  these  apparently  different 
kinds  of  matter.  To  take  one  concrete  example:  The  re- 
sults of  spectrum  analysis  show  that  most  of  the  chemical 
elements  have  their  own  definite  and  characteristic  spectrum. 

29 


30         THE  ELECTRICAL  NATURE  OF  MATTER 

That  an  element  sets  up  vibrations  in  the  ether  that  are  of 
perfectly  definite  wave-lengths,  and  by  means  of  which  the 
element  in  question  can  be  identified  —  these  being  different 
for  every  element. 

Further,  while  this  is  true,  certain  simple  and  beautiful 
relations  between  the  wave-lengths  of  the  waves  sent  out 
by  a  given  element  have  been  discovered. 

Thousands  of  facts  of  the  character  of  those  mentioned 
above  must  be  dealt  with  by  any  ultimate  theory  of  matter 
that  can  be  regarded  as  tenable. 

The  atomic  masses  of  the  chemical  atoms  are  as  different 
as  i.oi  for  hydrogen  and  238.5  for  uranium,  and  all  inter- 
mediate orders  of  magnitude  are  met  with.  These  masses 
are  due  wholly  or  in  part,  to  the  electrical  charges  or  elec- 
trons of  which  the  atoms  of  all  the  elements  are  composed. 

We  might  at  first  thought  conclude  that  the  atom  of  one 
element  differs  from  the  atom  of  another  element  only  in 
the  number  of  electrons  contained  in  it,  and  that  the  atoms 
are  simply  condensed  groups  or  nuclei  of  electrons. 

Such  a  conception  would  be  at  variance  with  the  facts 
of  both  chemistry  and  physics.  In  terms  of  such  a  con- 
ception, how  could  we  account  for  chemical  valency,  the 
acid-forming  property  of  some  elements  and  the  base- 
forming  property  of  others?  In  terms  of  such  a  condensa- 
tion conception  of  the  electrons,  how  should  we  account 
for  the  facts  of  spectrum  analysis? 

It  was  recognized  by  J.  J.  Thomson,  to  whom  we  owe 
the  entire  electron  conception,  that  we  cannot  do  so. 

It  is  true  that  the  atoms  with  different  atomic  masses 
must  have  different  numbers  of  electrons  in  them.  While 
this  is  a  necessary  condition,  it  is  far  from  sufficient  to 
account  for  the  facts  of  either  chemistry  or  physics. 


THE  ATOM  AND  THE  ELECTRON  THEORY  31 

THOMSON'S  CONCEPTION  OF  THE  ATOM 

The  electrons  are  moving  with  high  velocities  in  orbits 
within  the  atom,  occupying  a  relatively  small  part  of  the 
volume  occupied  by  the  atom  as  a  whole.  The  spaces 
between  the  electrons  in  an  atom  are  relatively  enormous, 
compared  with  the  spaces  occupied  by  the  electrons  them- 
selves. But  the  electrons  are  negative  electrical  charges, 
and  we  cannot  have  negative  electricity  without  the  corre- 
sponding positive.  Where  is  the  positive  electricity  corre- 
sponding to  these  negative  units? 

Thomson1  supposes  the  atom  to  be  made  up  of  a  sphere 
of  uniform  positive  electrification,  through  which  the  elec- 
trons or  negative  charges  are  distributed.  These  electrons 
are,  as  we  have  seen,  at  enormous  distances  apart  compared 
with  the  spaces  actually  occupied  by  them,  like  the  planets 
in  the  Solar  System;  and  move  with  very  high  velocities. 
The  corpuscles  are  so  distributed  through  the  positive 
sphere  as  to  be  in  dynamical  equilibrium  under  the  forces 
that  are  acting  upon  them.  These  are  the  attraction  of 
the  positive  electricity  for  the  negative  electrons,  and  the 
repulsion  of  one  negative  electron  by  another. 

This  brings  us  to  an  extremely  interesting  development 
of  the  electron  theory.  J.  J.  Thomson  has  solved  the 
problem,  in  part,  as  to  the  arrangement  of  the  corpuscles 
that  will  produce  stable  systems,  in  the  case  of  a  number 
of  the  less  complex  atoms. 

THE  ELECTRON  THEORY  AND  THE  PERIODIC  SYSTEM 

Thomson  has  calculated  the  arrangement  of  the  electrons 
in  a  sphere  of  positive  electrification,  which  will  be  stable. 
The  electrons  will  arrange  themselves  in  concentric  rings, 

Phil.  Mag.,  7,  237  (1904). 


32        THE  ELECTRICAL  NATURE  OF  MATTER 

since  a  large  number  of  corpuscles  arranged  in  a  single 
ring  cannot  be  stable.  This  ring,  however,  will  become 
stable  when  a  suitable  number  of  corpuscles  are  placed  in 
the  interior,  which  would  produce  a  system  with  concentric 
rings. 

In  the  following  table  is  given  the  total  numbers  of  elec- 
trons, in  which  the  outer  ring  will  contain  twenty,  and  also 
the  numbers  that  will  be  contained  in  the  inner  rings,  which 
are  four  in  number. 

NUMBER  OF  ELECTRONS 

59     60     61     62     63     64     65     66     67 

NUMBER  OF  ELECTRONS  IN  EACH  RING 
233334455 

8   8   9   9  10  10  10  10  10 

J3  J3  J3  T3  T3  *3  J4  14  J5 
16  16  16  17  17  17  17  17  17 

20       20       20       20       20       2O       2O       2O       2O 

The  smallest  number  of  electrons  which  will  have  an 
outer  ring  of  20  is  59,  and  the  largest  number  with  an  outer 
ring  of  20  is  67.  When  the  total  number  is  less  than  59, 
the  outer  ring  will  contain  less  than  20,  which  would  neces- 
sitate a  rearrangement  of  the  corpuscles.  If  an  electron 
was  removed  from  such  a  system,  the  system  would  of  neces- 
sity be  broken  down,  and  the  electrons  rearranged  in  a  new 
form,  which  would  be  the  stable  form  for  58  electrons. 
It  we  pass  to  the  other  extreme  of  the  systems  containing 
20  electrons  in  the  outer  ring,  we  shall  find  exactly  the  re- 
verse condition.  We  cannot  add  an  electron  to  this  system 
without  destroying  the  equilibrium.  If  an  electron  were 
added,  there  would  be  an  entire  rearrangement  of  the  whole 
system,  giving  us  a  new  system  with  21  electrons  in  the 


THE  ATOM  AND  THE  ELECTRON  THEORY       33 

outer   ring.     This    complete    breaking    up    of   the    system 
would,  of  course,  be  a  difficult  matter. 

Turning  now  to  the  systems  containing  total  numbers 
of  electrons  intermediate  between  59  and  67,  some  un- 
usually interesting  relations  manifest  themselves.  Take 
the  system  with  60  electrons.  One  electron,  and  only  one, 
can  be  detached  from  this  system  without  destroying  the 
equilibrium  and  necessitating  a  rearrangement  of  the  re- 
mainder. The  removal  of  one  electron  reduces  the  total 
number  to  59,  which,  as  we  have  seen,  is  the  smallest  num- 
ber that  is  stable  with  20  in  the  outer  ring.  Such  a  system 
having  lost  one  electron,  which  is  one  unit  of  negative  elec- 
tricity, would  be  electropositive. 

The  recent  study  of  chemical  valency  from  the  stand- 
point of  modern  physical  chemistry  has  shown  that  Fara- 
day's law  is  the  basis  of  all  chemical  valency.  This  means 
that  a  univalent  element  is  one  that  carries  unit  electrical 
charge,  a  bivalent  element  two  such  charges,  and  so  on.  In 
the  light  of  these  facts  we  see  that  the  above  system  with 
60  corpuscles,  having  lost  one  electron,  or  one  negative 
charge,  would  contain  one  positive  charge  in  excess,  and 
would,  therefore,  be  a  univalent  positive  element,  while  the 
system  with  59  corpuscles  would  have  no  valency. 

The  system  containing  61  electrons  could  lose  two  with- 
out destroying  the  equilibrium,  and  would,  therefore,  be 
a  bivalent,  positive  element. 

The  system  with  62  electrons  could  lose  three  without  de- 
stroying the  equilibrium,  and  would  correspond  to  a  triva- 
lent,  positive  element. 

If  now  we  pass  to  the  system  with  63  electrons,  we  can 
add  jour  electrons  without  increasing  the  total  number 
beyond  67,  and,  therefore,  without  destroying  the  stability 
of  the  system  as  a  whole  and  necessitating  a  rearrangement. 


34        THE  ELECTRICAL  NATURE  OF  MATTER 

Such  a  system  would  correspond  to  a  tetravalent  negative 
element. 

Similarly,  three  electrons  could  be  added  to  the  system 
where  the  total  number  is  64,  two  to  the  system  containing 
65,  and  one  to  the  system  containing  66,  without  destroying 
the  equilibrium.  These  would  then  correspond  respectively 
to  trivalent,  bivalent,  and  univalent  electronegative  elements. 

When  we  come  to  the  system  with  67  electrons,  we  find 
conditions  that  suggest  those  pointed  out  with  the  system 
with  59  electrons.  Just  as  in  the  latter  case  we  cannot 
remove  an  electron  without  destroying  the  equilibrium, 
just  so  when  we  have  67  electrons  we  cannot  add  an  elec- 
tron without  destroying  the  equilibrium  and  necessitating 
a  rearrangement  of  the  system  as  a  whole;  since,  it  will  be 
remembered,  that  67  is  the  largest  total  number  of  electrons 
that  can  have  an  outer  ring  of  20.  This,  like  the  system 
with  59  electrons,  would  correspond  to  an  element  with  no 
chemical  valency. 

Turning  now  to  the  Periodic  System,  we  find,  as  Thomson 
pointed  out,  that  the  first  nine  elements  are  the  following: 
Helium,  lithium,  glucinum,  boron,  carbon,  nitrogen,  oxygen, 
fluorine,  and  neon. 

The  second  series  of  nine  elements  is  the  following: 
Neon,  sodium,  magnesium,  aluminium,  silicon,  phos- 
phorus, sulphur,  chlorine,  and  argon. 

It  will  be  recognized  that  the  first  and  last  member  of 
each  of  the  above  series  has  no  valency,  since  they  have 
not  been  made  to  combine  chemically  with  anything  else. 
Lithium  and  sodium  are  univalent  elements  and  electro- 
positive, glucinum  and  magnesium  are  bivalent  and 
electropositive,  boron  and  aluminium  are  trivalent  and 
electropositive,  carbon  and  silicon  are  tetravalent 
and  electronegative,  nitrogen  and  phosphorus  trivalent 


THE   ATOM  AND   THE   ELECTRON   THEORY  35 

and  electronegative,  oxygen  and  sulphur  bivalent  and 
electronegative,  fluorine  and  chlorine  univalent  and  elec- 
tronegative, while  neon  and  argon  have  no  chemical 
valency  —  having  never  been  made  to  combine  with  any 
other  element.  A  more  perfect  agreement,  as  far  as  it 
goes,  between  the  deductions  from  any  theory  and  the 
facts  could  not  exist. 

Relations  such  as  the  above,  which  have  been  pointed 
out  by  Thomson,  have  done  much  to  bring  the  electron 
theory  of  matter  to  the  front,  and  are  altogether  too  com- 
prehensive to  be  attributed  to  accident.  This  application 
of  the  electron  theory  to  the  Periodic  System  is  one  of  the 
most  important  applications  of  this  conception  that  has 
thus  far  been  made. 

THE  ATOM  IN  TERMS  OF  THE  ELECTRON  THEORY 

The  atom  according  to  this  theory  is  very  complex.  Take, 
for  example,  the  atom  of  mercury.  This  contains  a  rela- 
tively large  number  of  electrons,  and  some  of  the  heavier 
atoms  are  even  more  complex.  The  approximate  number 
of  electrons  contained  in  an  atom  is,  according  to  recent 
views,  of  the  order  of  magnitude  of  its  atomic  weight. 

This  complex  nature  of  the  atoms  enables  us  to  account 
for  the  facts  of  spectrum  analysis.  Certain  elements,  such 
as  iron,  uranium,  etc.,  give  out  vibrations  of  thousands  of 
wave-lengths  in  the  ether,  in  accordance  with  the  prevail- 
ing theory  of  light;  as  is  shown  by  the  enormous  number 
of  spectrum  lines  produced  by  these  elements.  In  terms 
of  the  old  conception  of  the  atom,  it  was  difficult  to  see 
how  such  a  large  number  of  vibrations  of  such  widely 
different  periods  could  be  set  up  in  the  ether  by  a  single 
element.  Before  we  had  the  electron  theory,  it  was  recog- 
nized that  the  atom  must  in  its  ultimate  essence  be  complex, 


36        THE  ELECTRICAL  NATURE  OF  MATTER 

in  order  to  produce  such  effects  as  are  brought  out  by  spec- 
trum analysis  alone.  The  writer  has  heard  Rowland  fre- 
quently say,  that  the  simplest  atom  must  be  more  complex 
than  a  piano. 

The  electron  theory,  giving  us  some  idea  of  the  complexity 
of  even  the  simplest  atoms,  makes  it  possible  to  form  a 
mental  picture  of  how  an  atom  can  produce  such  effects 
in  the  ether  as  is  shown  by  a  study  of  the  spectrum. 

Light  is  not  only  thrown,  by  the  electron  theory,  on  the 
problem  of  spectrum  analysis,  but  on  a  host  of  similar 
problems,  which  it  would  lead  us  too  far  in  this  connection 
to  discuss. 

CATIONS  AND  ANIONS  IN  TERMS  OF  THE  ELECTRON  THEORY 

When  acids,  bases,  and  salts  are  dissolved  in  water  they 
break  down  into  a  positively  charged  constituent  known 
as  a  cation,  and  a  negatively  charged  constituent  known 
as  an  anion.  The  recognition  of  this  fact  is  one  of  the 
most  important  contributions  to  scientific  knowledge  made 
by  modern  physical  chemistry.  Before  we  had  the  elec- 
tron theory,  we  could  not  form  any  very  definite  mechanical 
conception  of  how  this  important  process  takes  place. 

We  knew  that  all  acids  yielded  the  hydrogen  cation, 
which  gave  their  solutions  acid  properties,  and  that  the  re- 
mainder of  the  molecule,  as  a  whole,  was  charged  negatively 
and  formed  the  anion  of  the  acid. 

We  also  knew  that  bases  dissociated  in  the  presence  of  a 
dissociating  solvent,  yielding  the  hydroxyl  anion  which  was 
characteristic  of  all  bases,  and  to  which  the  basic  properties 
are  due;  and  that  the  remainder  of  the  molecule  of  the 
base  became  charged  positively,  and  formed  the  cation  of 
the  base.  Just  as  all  acids  yield  the  hydrogen  cation,  so 
all  bases  yield  the  hydroxyl  anion. 


THE   ATOM  AND   THE   ELECTRON   THEORY  37 

We  knew,  further,  that  salts  in  the  presence  of  a  dis- 
sociating solvent,  break  down  or  dissociate,  as  we  say,  into 
a  cation  and  an  anion  —  the  cation  being  the  cation  of  the 
base  from  which  they  were  formed,  and  the  anion  the  anion 
of  the  acid  which  took  part  in  the  formation  of  the  salt. 

We  were,  however,  not  able  to  form  any  definite  con- 
ception of  how  certain  atoms  or  groups  (usually  atoms) 
became  charged  positively  and  thus  became  cations,  or 
how  certain  other  atoms  or  groups  (usually  groups  of  atoms) 
became  charged  negatively  and  thus  became  anions. 

The  electron  theory  solves  this  problem  in  a  very  satis- 
factory manner.  When  an  atom  loses  an  electron  it  becomes 
charged  positively,  since  the  loss  of  a  negative  charge  is 
exactly  equivalent  to  gaining  a  positive  charge.  Thus,  a 
cation  is  an  atom  or  group  of  atoms  that  has  lost  an  electron. 

If  an  atom  takes  on  an  electron  it  becomes  charged  nega- 
tively. An  anion  is  then  an  atom  or  a  group  of  atoms  that 
has  gained  an  electron. 

A  bivalent  cation  is  one  that  has  lost  two  electrons,  a 
trivalent  cation  is  one  that  has  lost  three  electrons,  and  so  on. 

A  bivalent  anion  is  one  that  has  gained  two  electrons,  a 
trivalent,  one  that  has  gamed  three  electrons,  and  so  on 
for  the  polyvalent  anions. 

Since  a  great  majority,  if  not  all  chemical  reactions  take 
place  between  ions,  and  since  electrons  are  so  vitally  con- 
nected with  the  formation  of  ions,  it  follows  that  the  electron 
theory  is  of  as  much  importance  for  the  science  of  chemis- 
try as  for  the  science  of  physics. 

THE  MASS  OF  AN  ION  NOT  EXACTLY  THE   SAME  AS  THAT  OF 
THE  ATOM  FROM  WHICH  IT  IS  FORMED 

From  the  above  method  of  ion  formation,  it  is  obvious 
that  the  mass  of  an  ion  is  different  from  that  of  the  atom  or 


38        THE  ELECTRICAL  NATURE  OF  MATTER 

group  of  atoms  from  which  it  was  formed.  Since  a  cation 
is  an  atom,  or  group  of  atoms,  from  which  one  or  more 
electrons  have  been  split  off,  a  cation  has  a  smaller  mass 
than  the  atom  or  atoms  from  which  it  was  produced. 

An  anion,  on  the  other  hand,  is  formed  from  an  atom  or 
group  of  atoms  by  adding  one  or  more  electrons.  There- 
fore, the  mass  of  an  anion  is  greater  than  the  mass  of  the  atom 
or  atoms  from  which  it  was  produced. 

It  must,  however,  be  remembered  that  the  difference 
between  the  mass  of  an  atom  or  group  of  atoms,  and  the 
corresponding  ion,  is  in  any  case  very  small.  Take  the 
hydrogen  atom  and  the  hydrogen  ion,  where  the  difference 
is  the  greatest.  The  hydrogen  atom  is  the  lightest  atom. 
The  loss  of  an  electron,  converting  the  hydrogen  atom 
into  the  hydrogen  cation,  would  change  the  mass  only 
about  rfW.  This  is  close  to  the  limit  of  accuracy  of 
our  most  refined  methods  of  measuring  mass,  and  it  is, 
therefore,  doubtful  whether  we  could  detect  the  difference 
between  the  mass  of  a  hydrogen  atom  and  the  correspond- 
ing hydrogen  ion  even  when  a  large  number  were  em- 
ployed. It  would,  however,  be  rash  to  assert  that  such 
differences  would  never  be  detected,  or  even  determined, 
by  using  a  very  large  number  of  hydrogen  atoms  and 
comparing  them  with  the  corresponding  ions. 

The  change  in  mass  would  be  relatively  less  for  any  other 
atom  when  it  is  converted  into  the  corresponding  ion,  since 
the  mass  of  any  other  atom  is  so  much  greater  than  that  of 
the  hydrogen  atom,  and  the  absolute  gain  or  loss  in  mass 
would  be  the  same  for  any  other  univalent  ion,  as  for  hydro- 
gen —  a  loss  for  every  cation,  and  a  gain  for  every  anion. 
That  this  is  true  is  seen  from  the  fact  that  every  univalent 
ion  differs  in  mass  from  the  corresponding  atom  only  in 
containing  one  more  or  one  less  electron. 


THE   ATOM   AND   THE   ELECTRON   THEORY  39 

The  same  remark  holds  for  polyvalent  ions,  which  differ 
from  the  corresponding  atoms  or  groups  of  atoms  in  that 
they  contain  a  number  of  electrons  greater  or  less  than 
the  corresponding  atoms,  expressed  by  the  valency  of  the 
ion  in  question.  The  mass  of  all  such  ions  is,  however,  so 
much  greater  than  that  of  the  hydrogen  ion,  that  if  we 
divide  their  mass  by  their  valency,  the  result  is  still  many 
times  greater  than  the  mass  of  the  hydrogen  ion.  The 
greatest  change  in  mass  is,  therefore,  that  produced  when  a 
hydrogen  atom  loses  an  electron  and  passes  over  into  the 
hydrogen  ion. 

Whether  or  not  this  change  in  mass  can  ever  be  detected 
directly,  it  is  important  to  recognize  that  the  mass  does 
change  whenever  an  atom  or  group  of  atoms  passes  over 
into  ions.  There  is  a  gain  in  the  mass  of  an  atom  whenever 
an  anion  is  formed  from  it,  and  a  loss  in  the  mass  of  an 
atom  whenever  a  cation  is  formed. 

It  must,  of  course,  be  remembered  that  a  cation  is  never 
formed  without  the  corresponding  anion  being  formed,  and 
vice  versa]  so  that  in  ionization  the  anion  gains  just  as  much 
in  mass  as  the  cation  loses,  and  the  total  mass  consequently 
remains  unchanged. 

When  a  molecule  of  an  electrolyte,  say  sodium  chloride, 
breaks  down  into  ions,  what  takes  place  is  the  transference 
of  an  electron  from  the  sodium,  which  becomes  a  cation,  to 
the  chlorine,  which  becomes  an  anion.  The  sodium  loses 
in  mass  an  amount  equal  to  the  mass  of  an  electron,  and  the 
chlorine  gains  the  same  amount  in  mass;  the  sum  of  the 
masses  of  sodium  and  chlorine  remaining  constant. 

There  would  be  a  change  in  the  total  masses  in  ioniza- 
tion only  if  we  assumed  that  there  was  a  change  in  the 
velocities  of  the  electrons  in  the  sodium  and  in  the  chlorine, 
when  ionization  takes  place,  and  that  these  changes  in  the 


4O.        THE  ELECTRICAL  NATURE  OF  MATTER 

velocities  did  not  exactly  compensate  one  another.  Since 
there  is,  at  present,  no  ground  for  such  an  assumption,  we 
must  conclude  that  the  total  masses  of  the  ions  formed 
from  any  molecule  are  equal  to  the  mass  oj  the  molecule. 

THE  ELECTRON  THEORY  AND  RADIOACTIVITY 

One  of  the  most  important  bearings  of  the  whole  electron 
theory  of  Thomson  is  in  connection  with  those  investiga- 
tions on  radioactivity  which  have  recently  attracted  so 
much  attention;  investigations  which  have  opened  up  an 
entirely  new  branch  of  experimental  physics,  and  which 
have  changed  some  of  our  fundamental  conceptions. 

The  application  of  the  electron  theory  to  these  epoch- 
making  investigations  will  be  made  when  these  researches 
are  studied. 

MORE  RECENT  VIEW  AS  TO  THE  NATURE  OF  THE  ATOM 

The  more  recent  view,  especially  of  Rutherford,1  as  to 
the  nature  of  the  atom  is  as  follows.  An  atom  consists  of 
a  very  small  central  core  of  positive  electricity,  sur- 
rounded by  electrons  or  negative  charges.  These  elec- 
trons as  a  whole,  have  a  negative  charge  which  is  just 
equal  to  the  positive  charge  of  the  core  about  which 
they  rotate. 

The  atom  contains  also  an  outer  system  of  electrons, 
which  are  held  much  less  firmly  than  the  inner  system. 
This  outer  system  gives  to  the  atom  its  characteristic 
physical  and  chemical  properties.  The  inner  system  of 
electrons  comes  into  play  in  producing  radioactivity. 

lPhil.  Mag.,  21,  669  (1911). 


CHAPTER  V 
THE  X-RAYS 

IN  I8Q5,1  a  paper  appeared  by  Rontgen,  then  of  Wurz- 
burg,  now  of  Munich,  "On  a  New  Kind  of  Radiation." 
It  was  announced  that  when  an  electric  discharge  is  passed 
through  a  Crookes  or  Lenard  tube,  which  is  nothing  but  a 
high- vacuum  tube,  there  was  given  off  from  the  tube  a  kind 
of  radiation  which  was  unknown  up  to  that  time,  and  which 
has  most  remarkable  properties.  Among  these  was  the 
property  of  great  penetrability.  The  radiation  passed 
through  objects  which  were  entirely  opaque  to  light,  and 
affected  a  photographic  plate.  When  a  photographic  plate 
was  covered  with  perfectly  black  paper,  or  placed  in  a  black 
wooden  box,  through  which  no  light  could  pass,  the  plate 
was  still  affected  by  the  newly  discovered  radiation.  In- 
deed, it  was  this  fact  that  led  to  the  discovery  of  the  radia- 
tion by  Rontgen. 

It  was  found  that  the  radiation  could  pass  through  a 
great  number  of  objects  that  were  entirely  opaque  to  light. 
Thus,  comparatively  thick  sheets  of  some  of  the  metals, 
such  as  aluminium,  were  quite  transparent  to  the  newly 
discovered  radiation.  It  had  the  power  of  passing  through 
metals  in  general;  but  the  heavy  metals,  such  as  lead, 
platinum,  and  the  like,  were  much  more  opaque  to  the 
radiation  than  the  lighter  metals.  It  was  soon  found  that 
the  bones  of  the  body  are  far  more  opaque  to  the  radia- 

1  Wied.  Ann.,  64,  i  (1898). 


42         THE  ELECTRICAL  NATURE  OF  MATTER 

tion  than  the  flesh,  and,  therefore,  photographs  of  the 
living  skeleton  could  be  obtained,  which  led  to  a  large 
amount  of  dilettanteism.  It  was  announced  that  the  radia- 
tion could  not  be  refracted,  nor  polarized.  When  passed 
through  a  gas  it  rendered  the  gas  a  conductor,  or,  as  we 
have  seen,  ionized  the  gas,  in  part. 

Of  course,  these  were  at  once  recognized  to  be  very  re- 
markable properties;  many  of  them  entirely  different  from 
those  of  any  known  form  of  radiation.  In  some  respects 
it  resembled  light,  but  in  most  of  its  properties  differed 
fundamentally  from  it. 

It  is  but  natural  that  such  a  discovery  should  have  awak- 
ened the  broadest  and  deepest  interest  on  the  part  of  men 
of  science,  the  world  over,  almost  regardless  of  the  branch  of 
natural  science  to  which  they  were  devoting  their  energies. 
The  first  question  that  would  naturally  be  asked  was,  What 
is  this  newly  discovered  kind  0}  radiation?  In  answering 
this  question  the  method  of  producing  the  radiation  must 
be  carefully  taken  into  consideration. 

NATURE  OF  THE  X-RAY 

It  will  be  seen  that  the  X-ray  is  produced  in  the  ordinary 
cathode  discharge  tube,  and  this  alone  would  serve  to  con- 
nect this  portion  of  the  work  with  what  has  preceded.  We 
have  already  studied  the  cathode  discharge,  and  the  velocity 
and  nature  of  the  cathode  particle.  We  now  see  that  a  re- 
markable kind  of  radiation  is  given  off  from  the  cathode  tube. 

Careful  study  showed  a  very  close  connection  between 
the  cathode  discharge  and  the  production  of  the  radiation. 
It  was  found  that  the  X-rays  were  produced  where  the 
cathode  rays  strike  upon  a  solid  body,  such  as  the  glass 
walls  of  the  low-pressure  tube.  The  cathode  rays  are 
thus  vitally  connected  with  the  production  of  the  X-rays. 


THE   X-RAYS  43 

Several  theories  have  been  advanced  to  account  for  the 
nature  of  the  new  radiation.  While  in  a  few  respects  it 
resembled  light,  in  most  of  its  properties  it  differed  funda- 
mentally from  light.  Light  is  a  transverse  vibration  of  the 
ether,  the  X-ray  might  be  a  longitudinal  vibration  in  the 
ether,  and  this  was  the  theory  that  was  proposed  by  Rontgen 
to  account  for  the  radiation  that  he  had  discovered.  As 
facts  accumulated,  this  theory  was  found  to  be  insufficient. 
Indeed,  it  never  acquired  any  prominence,  or  received  any 
very  serious  support.  It  remained  for  Sir  George  Stokes 
to  propose  a  theory  as  to  the  nature  of  the  X-ray  that 
would  prove  to  be  satisfactory,  and  account  for  the  facts 
then  known,  as  well  as  for  those  subsequently  to  be  dis- 
covered. (See  page  48.) 

The  X-ray  is  not  a  succession  of  waves  in  the  ether,  like 
light,  but  a  series  of  pulses  in  the  ether,  sent  out  at  irregular 
intervals.  This  was  in  accord  with  their  mode  of  formation, 
and  accounted  for  their  properties.  They  are  produced 
when  the  cathode  particles  in  a  cathode  discharge  fall  upon 
the  glass  walls  of  the  confining  vessel.  These  particles 
rain  down  upon  the  walls  of  the  tube  at  irregular  intervals, 
and  if  they  set  up  any  vibration  in  the  ether,  it  would  be 
expected  that  it  would  be  irregular  in  character. 

Further,  matter  would  be  supposed  to  be  far  more  trans- 
parent to  such  a  set  of  irregular  pulses,  than  to  a  definite, 
regular  set  of  vibrations  in  the  ether,  such  as  corresponds 
to  a  wave  of  light.  To  say  that  an  object  is  transparent  to 
any  given  form  of  radiation,  means  that  it  is  not  thrown  into 
vibration  by  the  radiation  when  the  radiation  falls  upon  it. 
On  the  other  hand,  to  say  that  an  object  is  opaque  to  a 
vibration,  means  that  it  is  thrown  into  vibration  by  the 
radiation.  Glass  is  transparent  to  light  because  it  is  not 
thrown  into  vibration  by  the  light.  A  thin  sheet  of  metal 


44          THE  ELECTRICAL  NATURE  OF  MATTER 

is  opaque  to  light  because  the  light  waves  falling  upon  it 
produce  vibrations  within  the  metal. 

This  is  just  what  we  should  expect,  since,  if  the  radiation 
sets  up  vibrations  in  the  object  upon  which  it  impinges,  its 
energy  is  expended  in  setting  up  the  vibrations,  and  the 
radiation  as  such  is  lost. 

The  penetrating  power  of  the  X-ray  is  thus  explained 
by  the  Stokes  theory  as  to  its  nature. 

Similarly,  this  theory  accounts  satisfactorily  for  the  other 
well- recognized  properties  of  the  X-ray,  and  is  now  gen- 
erally accepted. 

THE  BECQUEREL  RAY 

The  X-ray  is  produced,  as  we  have  seen,  where  the  cathode 
ray  falls  upon  the  wall  of  the  glass  tube.  It  will  be  re- 
membered, that  where  the  cathode  ray  falls  upon  the  wall 
of  the  tube  a  phosphorescent  spot  is  produced  on  the  glass. 
For  a  time  it  was  supposed  that  this  phosphorescence  is  in 
some  way  intimately  connected  with  the  production  of  the 
X-ray.  Although  it  has  subsequently  been  shown  that  this 
is  not  the  case,  and  that  X-rays  are  produced  better  when 
the  cathode  ray  falls  upon  a  metal  plate  which  does  not 
become  phosphorescent,  than  when  it  falls  upon  glass 
which  does;  yet  this  original  idea,  although  erroneous,  led 
to  highly  important  discoveries. 

With  the  idea  that  phosphorescence  and  X-rays  are  vitally 
connected,  men  of  science  began  to  examine  bodies  that 
were  naturally  phosphorescent,  to  see  whether  they  gave 
off  any  form  of  radiation  analogous  to  the  X-ray,  or  any 
unknown  form  of  radiation  whatsoever. 

It  remained  for  Henri  Becquerel1  to  discover  the  first 
naturally  radioactive  substance.  Guided  by  the  erroneous 

1  Compt.  rend.,  122,  501,  689,  and  762 


THE   X-RAYS  45 

idea  that  there  was  some  connection  between  the  phos- 
phorescence produced  on  the  glass  by  the  cathode  ray, 
and  the  production  of  the  X-ray  by  cathode  rays,  Becquerel 
began  examining  phosphorescent  substances  to  see  if  any 
of  them  gave  off  a  radiation  at  all  analogous  to  the  X-ray. 
He  chose  among  these  substances  the  salts  of  uranium, 
and  found  that  these  compounds  produced  an  impression 
on  a  photographic  plate  wrapped  in  black  paper  to  cut 
off  all  ordinary  light.  The  radiations  given  off  by  the 
salts  of  uranium  could  pass  through  thin  sheets  of  metal 
and  still  affect  the  photographic  plate. 

Becquerel  supposed  at  first  that  it  was  necessary  to  ex- 
pose the  phosphorescent  salts  of  uranium  to  sunlight,  in 
order  to  obtain  from  them  the  radiation  referred  to  above. 
He  found  later  that  this  radiation  was  given  off  even  when 
the  uranium  compound  had  not  previously  been  exposed 
to  light. 

Becquerel  tested  the  question,  as  to  whether  the  effect 
on  the  photographic  plate  was  due  to  any  volatile  substance 
given  off  from  the  uranium  salts.  This  was  especially 
desirable  in  the  light  of  the  recent  work  of  Russell,  on  sub- 
stances that  would  produce  a  fogging  of  photographic 
plates,  even  when  the  plate  was  not  directly,  but  only  in- 
directly, exposed  to  the  substances  in  question.  To  test 
this  point  the  photographic  plate,  wrapped  in  black  paper, 
was  screened  from  the  uranium  compound  by  a  thin  plate 
of  glass.  The  glass  would  have  cut  off  any  volatile  sub- 
stance given  off  from  the  compound  of  uranium.  The 
photographic  plate  was  still  affected,  which  showed 
that  the  result  was  not  due  to  any  volatile  substance  com- 
ing from  the  salt  of  uranium. 

Becquerel  found  that  all  the  salts  of  uranium  would  pro- 
duce the  effect,  both  those  that  are  phosphorescent,  and 


46        THE  ELECTRICAL  NATURE  OF  MATTER 

those  that  are  not.  The  phenomenon  was  thus  shown 
not  to  be  directly  connected  in  any  way  with  phosphores- 
cence. The  effect  produced  by  the  non-phosphorescent 
compounds  was  just  as  great  as  that  produced  by  those 
that  are  phosphorescent,  provided  that  they  were  taken 
in  quantities  that  contained  the  same  amount  of  uranium. 
The  phenomenon  was  therefore  due  to  the  uranium  itself. 
It  was  soon  shown  that  metallic  uranium  was  not  only 
active,  but  more  active  than  any  of  its  compounds. 

The  radiations  given  off  by  uranium,  either  in  the  ele- 
mentary state  or  in  its  compounds,  have  nothing  to  do 
with  its  previous  exposure  to  light.  When  the  metal  or 
its  compounds  are  kept  for  a  long  time  in  the  dark,  the 
intensity  of  the  radiation  is  undiminished.  It  is  thus  obvious 
that  the  energy  given  out  by  the  uranium  radiations  is  not 
derived  from  sunlight. 

Further,  the  intensity  of  the  radiation  given  out  by  ura- 
nium is  not  diminished  in  several  years,  i.e.,  during  the 
longest  time  over  which  observations  have  thus  far  been 
extended.  In  these  experiments  the  uranium  salts  were 
preserved  in  lead  boxes,  which  are  especially  opaque  to 
such  radiations  as  we  are  now  considering,  and  the  inten- 
sity of  the  radiations  measured  photographically  from  time 
to  time  without  removing  the  uranium  compound  from  the 
lead  box.  In  this  way  the  uranium  salt  was  never  exposed 
to  radiations  from  external  sources,  and  yet  it  continued  to 
give  off  radiations  with  undiminished  intensity.  The 
energy  of  the  uranium  radiation  is  thus  intrinsic  in  the 
uranium,  and  does  not  come  from  a.ny  external  source. 

This  property  of  substances  to  emit  radiations  naturally 
like  uranium,  without  any  external  cause,  is  known  as 
radioactivity,  and  such  substances  are  radioactive.  There 
are  a  number  of  such  substances,  as  we  shall  see. 


THE   X-RAYS  47 

PROPERTIES  OF  THE  BECQUEREL  RAY 

It  was  early  recognized  that  the  uranium  radiations,  like 
the  Rontgen  rays,  have  many  remarkable  properties.  As 
we  shall  see,  they  have  some  properties  in  common,  while 
others  are  quite  different. 

The  uranium  radiations,  like  the  X-ray,  have  the  property 
of  ionizing  gases  through  which  they  pass.  This  is  shown 
by  the  fact  that  they  discharge  electrified  bodies  surrounded 
by  the  gases  in  question.  The  gases  are  ionized  by  the 
radiations,  and  then  conduct  the  charges  away  from  the 
charged  bodies  with  which  they  come  in  contact. 

In  this  respect,  as  well  as  in  their  power  to  affect  a  photo- 
graphic plate,  the  uranium  rays  act  like  the  X-ray,  but  they 
are  very  much  weaker  in  their  action.  ,This  applies  both 
to  their  action  on  a  photographic  plate,  and  their  power  to 
ionize  a  gas.  From  these  facts  alone  it  might  be  concluded 
that  the  Becquerel  ray  is  nothing  but  a  very  weak  form  of 
X-ray.  * 

The  rays  from  uranium  can  neither  be  refracted  nor 
polarized,  and  thus  again  resemble  the  X-ray. 

THE  THORIUM  RADIATION 

After  Becquerel  had  shown  that  one  natural  substance  is 
radioactive,  or  has  the  power  of  giving  out  radiations  that 
can  pass  through  considerable  thicknesses  of  matter  opaque 
to  light,  as  well  as  ionize  a  gas  and  affect  a  photographic 
plate,  a  search  was  made  for  other  natural  substances 
having  the  same  properties.  The  first  one  discovered  was 
the  comparatively  rare  element  thorium.  Schmidt *  found 
that  thorium,  whether  elementary  or  in  combination,  had 
some  properties  analogous  to  those  possessed  by  uranium. 
It  gave  out  radiations  that  acted,  if  only  feebly,  upon  the 

1  Wied.  Ann.,  65,  141  (1898). 


48        THE  ELECTRICAL  NATURE  OF  MATTER 

photographic  plate.  It  ionized  a  gas,  like  the  radiations 
from  uranium,  but  possessed  properties  that  distinguished 
it  sharply  from  the  uranium  radiation.  There  is  given  off 
from  the  thorium  something  that  is  blown  about  by  the 
slightest  currents  of  air,  and  which  in  some  respects  re- 
sembles a  gas.  This  was  discovered  by  Rutherford  and 
termed  by  him  an  emanation.  As  we  shall  learn,  this 
emanation  has  remarkable  properties. 

RECENT   WORK  ON   THE   NATURE   OF   THE   X-RAY 

The  recent  work  of  Laue,  Bragg  and  others,  has  changed 
our  conception  as  to  the  nature  of  the  X-ray.  If  the  rays 
were  a  regular  series  of  vibrations  in  the  ether,  with  wave- 
lengths say  of  molecular  dimensions,  when  allowed  to  fall 
on  a  grating  with  a  distance  between  the  lines  also  of 
molecular  dimensions,  we  would  have  produced  an  X-ray 
spectrum. 

A  crystal  is  just  such  a  space-grating.  When  X-rays 
are  reflected  from  a  crystal,  we  have  produced  spectra  of 
various  orders. 

From  the  positions  of  the  spectra  of  the  various  orders, 
and  the  intermolecular  distances  in  the  crystal,  we  can 
calculate  the  approximate  wave-lengths  of  the  X-rays. 
These  wave-lengths  vary,  but  are  of  the  magnitude  of  an 
Angstrom  unit. 

This  means  that  X-rays  are  not  a  series  of  irregular  pulses 
in  the  ether,  as  Stokes  supposed,  but  like  light  a  series  of 
regular  vibrations.  The  X-rays  differ  from  light  in  that 
the  lengths  of  the  ether  waves  are  much  less.  This  explana- 
tion of  the  nature  of  the  X-ray  is  in  harmony  with  its 
properties. 


CHAPTER  VI 
THE  DISCOVERY  OF  RADIUM 

IT  having  now  been  shown  that  two  elementary  sub- 
stances, uranium  and  thorium,  are  radioactive,  a  large 
number  of  substances  were  examined  with  respect  to  this 
property.  Among  these  would  naturally  be  the  minerals 
in  which  uranium  and  thorium  occur. 

Mme.  Curie1  determined  the  radioactivity  of  a  large 
number  of  minerals,  by  measuring  the  conductivity  of  the  air 
when  exposed  to  these  substances.  She  found  that  all  min- 
erals which  show  radioactivity  contain  either  uranium  or 
thorium.  What  was  very  remarkable  was  the  fact  that  certain 
minerals  which  contain  many  things  in  addition  to  uranium 
were  much  more  radioactive  than  uranium  itself.  Thus,  pitch- 
blende from  Johanngeorgenstadt  had  nearly  four  times  the 
radioactivity  of  pure  uranium.  Pitchblende  from  Joachims- 
thai  was  three  times  as  radioactive  as  uranium,  while  pitch- 
blende from  Pzibran  was  nearly  three  times  as  radioactive. 

Chalcolite,  which  is  a  double  phosphate  of  copper  and 
uranium,  is  about  two  and  one-fourth  times' as  radioactive 
as  metallic  uranium,  while  autunite,  a  double  phosphate 
of  calcium  and  uranium,  is  about  one  and  one-fifth  times 
as  radioactive  as  uranium. 

Only  a  part  of  every  one  of  these  minerals  is  uranium, 
and  yet  the  mineral  was  more  radioactive  than  pure  ura- 
nium itself. 

Mme.  Curie  then  prepared  chalcolite  artificially  by  treat- 

1  Ann.  Chim.  Phys.  [7],  30,  99  (1903). 
49 


50        THE  ELECTRICAL  NATURE  OF  MATTER 

ing  a  solution  of  uranyl  nitrate  with  a  solution  of  copper 
phosphate  in  phosphoric  acid,  and  warming  the  mixture 
to  fifty  or  sixty  degrees.  Under  these  conditions  crystals 
of  chalcolite  were  formed. 

The  radioactivity  of  this  artificially  prepared  chalcolite 
was  two  and  one-half  times  smaller  than  that  of  uranium 
itself.  This  led  Mme.  Curie  to  conclude  that  the  unex- 
pectedly great  activity  of  the  natural  minerals  was  due  to 
the  presence  in  them  of  small  quantities  of  some  strongly 
radioactive  substance,  which  was  neither  uranium,  nor 
thorium,  nor  any  other  known  substance. 

With  this  idea  in  mind  M.  and  Mme.  Curie  undertook 
to  separate  from  the  uranium  minerals  the  supposed  new 
radioactive  substance,  and  with  signal  success. 

THE  SEPARATION  OF  RADIUM  FROM  PITCHBLENDE 

Pitchblende,  as  is  well  known,  contains,  in  addition  to 
uranium,  a  large  number  of  other  elements  in  small  quantities. 
The  separation  of  pitchblende  into  its  constituents,  or  even 
the  separation  of  any  constituent  in  pure  form,  is  not  likely  to 
be  a  simple  matter.  The  Curies,  however,  worked  out  a 
chemical  method  for  effecting  the  desired  separation,  and 
obtaining  the  highly  radioactive  substance  or  substances. 

In  the  various  chemical  processes  to  which  the  material, 
as  we  shall  see,  was  subjected,  they  followed  the  course  of 
the  radioactive  constituents  by  determining  the  radioactivity 
of  every  product  by  means  of  the  electroscope.  They  could 
thus  determine  what  chemical  operation  was  concentrating 
the  radioactive  substance. 

There  are  at  least  two,  and  possibly  three  radioactive 
constituents  in  pitchblende,  in  addition  to  uranium  itself. 
One  of  these,  called  polonium  from  the  native  country 
(Poland)  of  Mme.  Curie,  resembles  in  its  chemical  proper- 


THE  DISCOVERY  OF  RADIUM  51 

ties  the  element  bismuth,  and  is  separated  from  the  pitch- 
blende along  with  this  element.  The  element  radium, 
with  which  we  are  now  chiefly  concerned,  is  closely  allied 
chemically  to  barium,  and  comes  out  of  the  pitchblende 
along  with  the  barium. 

A  third  radioactive  substance,  actinium,  has  been  de- 
scribed by  Debierne  as  occurring  in  pitchblende.  It 
seems  to  separate  from  pitchblende  along  with  certain  of 
the  rare  elements,  and  especially  thorium. 

To  give  some  idea  of  the  number  and  complexity  of  the 
chemical  processes  involved  in  separating  radium  from 
pitchblende,  the  essential  features  in  Mme.  Curie's l  account 
of  her  own  work  are  appended.  All  of  the  new  radioactive 
constituents  occur  in  pitchblende  in  minute  quantities,  so 
that  it  is  necessary  to  work  over  enormous  quantities  of 
material  in  order  to  obtain  even  a  few  milligrams  of  the 
comparatively  pure  radioactive  substances. 

We  shall  confine  our  account  to  the  separation  of  radium 
from  pitchblende,  which,  we  will  remember,  comes  out 
along  with  the  barium,  to  which  it  is  so  closely  related 
chemically. 

The  finely  powdered  pitchblende  is  fused  with  sodium 
carbonate,  and  the  product  treated  with  hot  water.  Dilute 
sulphuric  acid  is  then  added.  The  uranium  is  contained 
in  the  solution,  and  since  the  pitchblende  was  worked  for 
the  uranium  that  it  contained,  the  residue,  after  the  above 
treatment,  was  discarded.  The  radioactive  constituents 
are  contained  in  this  residue,  which  has  a  radioactivity  of 
about  4.5  times  that  of  metallic  uranium. 

This  residue  consists  mainly  of  the  sulphates  of  lead  and 
calcium.  It  also  contains  aluminium,  iron,  silicon,  and 
larger  or  smaller  amounts  of  nearly  all  known  metals.  The 

1  Ann.  Chim.  Phys.  [7],  30,  125-127. 


52         THE  ELECTRICAL  NATURE  OF  MATTER 

radium  exists  in  this  mixture  of  sulphates,  its  sulphate 
being  the  least  soluble. 

The  problem  now  is  to  separate  the  radium  from  this  mix- 
ture of  sulphates.  The  residue  is  freed  as  far  as  possible 
from  sulphuric  acid,  by  treating  with  a  concentrated,  boiling 
solution  of  sodium  hydroxide.  The  sulphates  of  calcium, 
aluminium,  and  lead  are  thus,  for  the  most  part,  decomposed, 
the  sodium  hydroxide  also  removing  the  aluminium,  silicon, 
and  lead.  The  residue  insoluble  in  the  alkali  is  washed  with 
water  and  then  treated  with  hydrochloric  acid.  The  radium 
remains  in  the  residue  insoluble  in  hydrochloric  acid. 

The  insoluble  portion  containing  the  radium  is  washed 
with  water,  and  then  treated  with  a  concentrated,  boiling 
solution  of  sodium  carbonate.  This  transforms  the  sul- 
phates of  barium  and  radium  into  carbonates.  The  car- 
bonates are  now  thoroughly  washed  with  water  and  treated 
with  hydrochloric  acid,  when  the  barium  and  radium  dis- 
solve as  the  corresponding  chlorides.  The  radium  is  pre- 
cipitated by  means  of  sulphuric  acid.  The  precipitate  also 
contains  barium  and  calcium,  lead  and  iron.  This  is  the 
radium-containing  barium  in  the  form  of  crude  sulphate. 

From  a  ton  of  the  residue  obtained  from  pitchblende,  ten 
or  twenty  kilograms  of  the  crude  sulphate,  having  an  activity 
from  thirty  to  sixty  times  that  of  metallic  uranium,  which 
is  taken  as  unity,  can  be  obtained. 

The  mixture  of  crude  sulphates  is  boiled  with  a  solution 
of  sodium  carbonate,  and  then  transformed  into  chlorides 
by  treating  the  carbonates  with  hydrochloric  acid.  The 
oxides  and  hydroxides  are  precipitated  by  adding  ammonia 
after  filtering.  Sodium  carbonate  is  added  to  the  solution, 
when  the  carbonates  of  the  alkaline  earths  are  thrown  down. 
The  carbonates  are  transformed  into  chlorides  by  adding 
hydrochloric  acid,  and  the  chlorides,  after  evaporating  the  so- 


THE   DISCOVERY   OF   RADIUM  53 

lution  to  dryness,  are  treated  with  pure,  concentrated  hydro- 
chloric acid.  This  dissolves  the  chloride  of  calcium,  while  the 
chlorides  of  barium  and  radium  are  insoluble  in  the  acid. 

About  eight  kilograms  of  this  product,  consisting  mostly 
of  barium  chloride,  are  obtained  from  a  ton  of  the  original 
residue.  The  radium  chloride  is  mixed  in  small  quan- 
tity with  the  barium  chloride.  This  is  shown  by  the  fact 
that  the  activity  of  the  radium-bearing  chloride  is  about 
sixty  times  that  of  pure  uranium. 

The  process  of  preparing  pure  radium  chloride,  instead 
of  being  ended,  is  now  really  only  begun.  The  following 
process  of  obtaining  radium  chloride  from  the  mixture  with 
barium  chloride  is  described  by  Mme.  Curie.1 

The  principle  of  the  method  is  fractional  crystallization. 
The  chloride  of  radium  is  less  soluble  than  the  chloride  of 
barium. 

The  first  fractionation  is  effected  in  pure  water.  The 
chloride  that  separates  from  the  saturated  solution  is  much 
more  active  than  the  solution,  as  would  be  expected,  since 
the  chloride  of  radium  is  less  soluble  than  the  chloride  of 
barium.  By  utilizing  this  fact,  and  fractionating  the  mix- 
ture in  terms  of  it,  after  a  long  series  of  fractionations,  dis- 
carding the  weakly  active  portions,  most  of  the  inactive 
barium  chloride  is  removed. 

When  a  large  number  of  fractionations  have  been  made,  and 
the  amount  of  substance  has  become  small,  it  is  better  to  add 
hydrochloric  acid  to  the  water,  since  this  diminishes  the  solu- 
bility of  the  salts,  and  more  rapid  separations  are  effected. 

Mme.  Curie  observed  that  the  crystals  of  radium  barium 
chloride  remain  colorless,  until  the  amount  of  radium  has 
reached  a  certain  per  cent,  of  the  whole  mass.  When  the 
radium  salt  has  reached  a  certain  concentration,  the  crys- 

1  Ann.  Chim.  Phys.  [7],  30,  131  (1903). 


54        THE  ELECTRICAL  NATURE  OF  MATTER 

tals  become  yellow.  They  may  even  show  an  orange,  or 
a  beautiful  rose  color.  This  color  possessed  by  the  crystals 
disappears  when  the  crystals  are  dissolved.  The  appear- 
ance of  this  color  is  rather  remarkable,  since  crystals  of 
pure  radium  chloride  are  colorless.  The  color  indicates 
that  a  certain  degree  of  purity  has  been  reached,  and  has  a 
maximum  intensity  when  the  amount  of  radium  present  is  a 
certain,  definite  quantity.  After  this  concentration  is  reached, 
the  intensity  of  the  color  becomes  less  and  less  as  the  purity 
of  the  crystals  becomes  greater  and  greater.  When  the 
radium  has  become  freed  from  all  appreciable  quantities 
of  barium,  the  color  practically  disappears  from  the  crystals. 
Thus,  the  color  of  the  crystals  can  be  used  as  an  index  to 
the  progress  of  the  separation  of  barium  from  the  radium 
-  of  the  degree  of  purity  of  the  radium  salt.  For  a  more 
detailed  discussion  of  these  matters  see  the  original  article 
by  Mme.  Curie. 

By  the  above  described  method  radium  chloride  can  be 
obtained,  having  a  radioactivity  that  is  one  million  times 
that  of  the  mineral  from  which  it  came. 

When  we  consider  the  number  of  steps  in  the  above 
described  process,  and  the  details  of  every  step,  and  then 
remember  that  every  one  of  these  details  had  to  be  worked 
out  empirically  by  the  Curies,  we  gain  some  idea  of  the 
enormous  task  that  they  have  performed,  and  the  difficul- 
ties at  every  step  which  they  must  have  encountered  and 
have  overcome. 

After  all  this  had  been  done,  the  amount  of  radium  chloride 
obtained  from  a  ton  of  the  residues  from  pitchblende  was 
only  a  few  milligrams.  This  necessitated  the  working 
over  of  enormous  quantities  of  the  original  pitchblende,  in 
order  to  obtain  any  appreciable  quantity  of  the  radium  salt. 
Fortunately,  this  problem  is  rendered  much  less  difficult 


THE  DISCOVERY   OF  RADIUM  55 

than  it  would  otherwise  be,  by  the  cooperation  on  the  part 
of  the  factories  in  which  pitchblende  is  used.  Many  of 
the  steps  described  in  the  above  process  can  be  taken  more 
successfully  on  a  large  scale  than  on  a  small  one,  to  say 
nothing  of  the  amount  of  time  and  labor  that  it  would  be 
necessary  to  expend  in  performing  these  operations  in  the 
laboratory.  Indeed,  if  we  were  dependent  upon  the  labora- 
tory alone  for  our  supply  of  radium,  our  knowledge  of  this 
substance  would  have  accumulated  infinitely  more  slowly 
than  it  has  done. 

Other  methods  have  been  proposed  for  purifying  the 
radium  salt,  which  are  hardly  more  than  modifications  of 
certain  details  of  the  method  worked  out  by  the  Curies  and 
described  above. 

The  question  that  arises  is  whether  some  source  of  radium 
richer  than  pitchblende  may  not  yet  be  found.  Radium 
has  been  shown  to  be  very  widely  distributed  over  the  sur- 
face of  the  earth.  It  occurs  in  a  large  number  of  minerals, 
in  the  waters  of  many  springs,  in  the  soil  and  rocks,  and 
probably  in  many  places  not  yet  discovered.  While  sources 
of  radium  that  are  richer  in  this  substance  than  the  richest 
pitchblendes  may  yet  be  found,  it  appears  to  the  writer  to 
be  doubtful  whether  any  material  very  rich  in  radium  will 
ever  be  found. 

This  opinion  is  not  based  so  much  upon  the  ease  with 
which  radium  is  detected  by  means  of  the  electroscope, 
or  upon  the  comparatively  wide  search  that  has  already 
been  made  for  this  substance,  as  it  is  upon  the  instability 
of  the  element  itself. 

As  we  shall  see,  radium  is  not  a  stable  substance.  It  is 
continually  undergoing  decomposition  into  other  things. 
It  would,  therefore,  be  very  surprising  if  any  large  quantity 
of  it  should  be  found  in  any  one  locality. 


56  THE  ELECTRICAL  NATURE   OF   MATTER 

THE  SPECTRUM  OF  RADIUM 

Since  radium  has  a  well-defined  spectrum,  it  is  a  matter 
of  great  importance  in  connection  with  the  determination 
of  the  purity  of  any  given  sample  of  its  salts.  To  deter- 
mine the  spectrum,  the  Curies1  turned  over  to  Demarcay 
some  samples  of  material  containing  radium,  and  he  studied 
the  spark  spectra  of  these  substances.  The  first  sample 
used  by  Demarcay  contained  large  quantities  of  barium. 
Nevertheless,  even  with  this  material  he  was  able  to  recog- 
nize, in  addition  to  the  barium  lines,  a  line  in  the  ultra- 
violet, having  a  wave  length  of  3814.7  Angstrom  units. 
When  a  purer  substance  was  used  the  intensity  of  this  line 
increased,  and  other  lines  made  their  appearance.  Finally, 
a  product  was  obtained  of  such  purity  that  only  the  three 
strongest  barium  lines  appeared  at  all,  and  -these  were  of 
such  slight  intensity  as  to  show  that  the  barium  was  pres- 
ent only  in  very  small  quantity.  While  this  product  was 
nearly  pure  radium  chloride,  it  was  still  further  purified 
until  the  strongest  barium  lines  could  scarcely  be  detected 
at  all. 

The  chief  lines  of  radium  found  by  Demarcay,  lying 
between  5,000  and  3,500,  are  the  following  —  the  most 
intense  line  being  represented  by  the  number  16. 

Wave  Length  Intensity 

4826.3  10 

4683.0  14 

4533-5  9 

4436.1  8 

4340.6  12 

3814.7  16 
3649.6  12 

1  Ann.  Chim.  Phys.  Ty],  30,  121  (1903). 


THE   DISCOVERY   OF   RADIUM  57 

The  strongest  of  the  above  lines  have  the  intensity  of 
the  stronger  lines  of  other  substances. 

In  addition  to  the  lines  referred  to  above,  and  a  number  of 
weaker  lines,  the  spectrum  of  radium  contains  two  bands;  the 
one  extending  from  4631.0  to  4621.9,  the  other  a  stronger 
band  in  the  ultraviolet,  extending  from  4463.7  to  4453.4. 

Thus,  the  spectrum  of  radium  resembles  the  spectra  of  the 
alkaline  earths,  which  consist  of  strong  lines  and  also  bands. 

It  was  pointed  out  by  Mme.  Curie  that  although  spectrum 
analysis  is,  in  general,  a  very  sensitive  means  of  detecting 
minute  quantities  of  substances,  in  the  case  of  radium  it  is 
far  less  delicate  than  the  electrometer,  notwithstanding  the 
fact  that  radium  gives  a  well-defined  spectrum. 

In  order  to  photograph  the  strongest  spectrum  lines  of 
radium,  a  specimen  of  the  radium- containing  barium  was 
required,  which  had  an  activity  at  least  fifty  times  that  of 
metallic  uranium. 

A  very  sensitive  electrometer,  on  the  other  hand,  can  de- 
tect radium  which  has  an  activity  that  is  only  one  ten- 
thousandth  that  of  metallic  uranium.  The  electrical  method 
of  detecting  the  presence  of  traces  of  radium  is  thus  at  least 
five  hundred  thousand  times  more  sensitive  than  the  spec- 
troscopic.  The  spectroscopic  method  is  of  importance  in 
connection  with  the  study  of  radioactivity,  not  so  much  as 
a  method  for  measuring  radioactivity,  as  for  determining 
the  purity  of  the  radium  in  the  various  stages  of  its  separa- 
tion from  barium.  Radium  bromide  gives  a  deep- red 
color  to  the  flame. 

THE  ATOMIC  WEIGHT  OF  RADIUM 

The  atomic  weight  of  radium  was  first  determined  by 
Mme.  Curie,1  with  specimens  that  contained  more  or  less 

1  Ann.  Chim.  Phys.  [7],  30,  137  (1903). 


58         THE  ELECTRICAL  NATURE  OF  MATTER 

barium.  Values  as  low  as  140  were  at  first  obtained.  As 
purer  and  purer  specimens  were  prepared,  successive  deter- 
minations gave  larger  and  larger  values  for  the  atomic 
weight  of  radium. 

A  specimen  which  still  showed  the  strongest  lines  of 
barium  with  appreciable  intensity,  gave  a  value  for  the 
atomic  weight  of  radium  ranging,  for  five  determinations, 
between  220.7  and  223.1. 

A  specimen  of  radium  chloride  was  then  purified  until 
the  strongest  lines  of  barium  appeared  very  weak  indeed. 
From  the  minute  quantity  of  barium  that  can  be  detected 
by  the  spectroscope,  this  specimen  of  radium  chloride  could 
contain  only  the  merest  trace  of  barium. 

The  atomic  weight  determinations  were  made  by  precipi- 
tating the  chlorine  as  silver  chloride.  Taking  the  atomic 
weight  of  silver  as  107.8  and  chlorine  as  35.4,  the  atomic 
weight  of  radium  was  found  to  be  225,  ranging  in  three 
determinations  between  224.0  and  225.8. 

Light  has  been  thrown  on  the  atomic  weight  of  radium 
by  Runge  and  Precht,1  who  studied  the  spectrum  of  radium 
in  a  magnetic  field.  Series  of  lines  were  observed,  with 
radium,  under  these  conditions,  that  are  analogous  to  those 
found  for  the  alkaline  earth  metals  —  calcium,  strontium, 
and  barium.  Certain  relations  have  been  established 
between  the  series  of  lines  for  an  element,  and  its  atomic 
weight.  By  means  of  these  relations  Runge  and  Precht 
have  calculated  the  atomic  weight  of  radium  to  be  257.8. 
However,  other  investigators,  and  especially  Watts,  on  purely 
physical  grounds,  have  concluded  that  the  atomic  weight  is 
close  to  225.  We  must,  then,  decide  between  these  two 
numbers.  In  the  light  of  the  evidence  at  present  available, 
this  is  not  an  easy  task. 

1  Phil.  Mag.,  5,  476  (1903). 


THE   DISCOVERY   OF   RADIUM  59 

The  'number  225  seems  to  fall  in  with  the  value  that 
radium  might  be  expected  to  have  from  the  Periodic  System. 
This  number  would  place  radium  after  bismuth  with  an 
atomic  weight  of  208.5  and  before  thorium  with  an  atomic 
weight  of  232.5  The  number  225  for  its  atomic  weight 
would  place  radium  in  group  II,  along  with  calcium,  stron- 
tium, and  barium,  to  which  chemically  it  is  closely  allied  — 
especially  to  barium,  as  we  have  seen.  The  atomic  weight 
225  also  places  it  in  the  twelfth  series,  along  with  thorium 
and  uranium  —  the  other  well-known  elements  that  are 
radioactive. 

On  the  other  hand,  the  atomic  weight  225  places  radium 
in  the  second  group  of  the  Periodic  System,  while  thorium 
is  in  the  fourth,  and  uranium  is  in  the  sixth.  In  a  word, 
it  places  radium  before  thorium  and  uranium;  the  atomic 
weight  of  thorium  being  232.5,  and  that  of  uranium  238.5. 
It  will  be  observed  that  these  three  radioactive  elements 
have  the  largest  atomic  weights  of  all  the  known  chemical 
elements.  Indeed,  an  attempt  has  been  made  to  establish 
a  relation  between  the  relatively  large  masses  of  the  atoms 
of  these  elements  and  their  radioactivity  —  an  attempt 
which,  as  we  shall  see  when  we  come  to  study  the  nature 
of  radioactivity,  is  most  praiseworthy.  In  terms  of  this 
relation,  the  atom  with  the  largest  mass  should  be  the  most 
radioactive,  and  as  we  usually  measure  mass  by  weight, 
the  atom  with  the  largest  atomic  weight  should  be  the  most 
radioactive.  If  the  atomic  weight  of  radium  is  257.8,  it 
would  be  in  accord  with  this  relation.  The  atom  of  radium 
would  be  by  far  the  heaviest  of  all  known  atoms,  that  of 
uranium  with  a  mass  of  238.5  would  be  next,  followed  by 
thorium  with  a  mass  of  232.5. 

We  shall  see  later  the  significance  of  this  relation,  and 
will  become  so  impressed  by  it  in  connection  with  the  appli- 


60        THE  ELECTRICAL  NATURE  OF  MATTER 

cation  of  the  electron  theory  of  matter  to  the  explanation 
of  radioactivity,  that  we  shall  be  loath  to  give  it  up,  and 
accept  a  lower  atomic  weight  for  radium  than  for  uranium 
and  thorium. 

Since  writing  the  above  a  relation  has  appeared  to  the 
writer,1  which  somewhat  invalidates  the  argument  for  225 
as  the  atomic  weight  of  radium,  based  upon  the  Periodic 
System.  If  we  turn  to  the  Periodic  System  and  examine 
the  atomic  weights  of  any  two  elements  in  the  same  group 
and  in  two  succeeding  series;  in  a  word,  of  two  elements 
that  fall  directly  under  one  another,  we  find  that  their 
atomic  weights  differ  from  one  another  by  from  twenty-five 
to  thirty  units.  This  is  especially  true  for  the  elements 
with  higher  atomic  weights.  Take  the  members  of  group 
II,  in  which  radium  undoubtedly  belongs  chemically.  The 
atomic  weight  of  calcium  is  40.1,  that  of  zinc  65.4  —  dif- 
ference 25.3.  Zinc  differs  from  strontium  in  round  num- 
bers by  twenty- two  points;  strontium  from  cadmium  by 
twenty-five  points,  and  cadmium  from  barium  by  twenty- 
five  points.  Yttrium  differs  from  indium  by  twenty-six, 
units;  indium  differs  from  lanthanum  by  twenty-four  units; 
lanthanum  differs  from  ytterbium  by  thirty-four  units,  and 
ytterbium  differs  from  thallium  by  thirty-one  units.  Simi- 
lar relations  exist  between  successive  members  of  every 
other  group  in  the  Periodic  System,  especially  between  the 
members  with  the  higher  atomic  weights. 

It  will  be  seen  that  the  difference  between  the  atomic 
weight  of  radium  as  determined  by  chemical  analysis  (225), 
and  as  determined  by  spectrum  analysis  (257.8),  is  about 
'thirty- three  units. 

We  have  already  seen  that  the  number  225  places  radium 
in  group  II  of  the  Periodic  System,  and  in  series  twelve. 

1  Amer.  Chem.  Journ.,  34,  467  (1905). 


THE   DISCOVERY   OF   RADIUM  6 1 

The  atomic  weight  256  to  258  would  place  radium  in  group 
II  of  the  Periodic  System,  and  in  series  thirteen.  This 
may  seem  surprising  since  only  twelve  series  have  thus  far 
been  recognized  in  the  Periodic  System.  It  may  be  that 
the  proper  place  for  radium  is  in  a  new  series,  of  which  only 
one  number  exists,  or  has,  at  least,  thus  far  been  discovered. 
If  radium  has  an  atomic  weight  of  258,  or  thereabouts,  it 
would  thus  fall  in  group  II  of  the  Periodic  System,  with  its 
chemically  allied  elements,  just  as  well  as  if  it  had  an  atomic 
weight  of  225.  The  fact  that  258  places  radium  on  the 
right-hand  side  of  group  II  is  not  a  serious  objection  to  the 
above  view,  since  we  do  not  know  that  the  relations  within 
the  groups  hold  for  these  highest  atomic  weights. 

The  problem  of  the  atomic  weight  of  radium,  however, 
cannot  be  settled  by  reasoning  from  analogy,  but  must  be 
worked  out  by  some  direct  method. 

If  we  examine  the  method  employed  by  Mme.  Curie  for 
determining  the  atomic  weight  of  radium,  it  does  not  seem 
to  be  entirely  free  from  objections.  In  the  first  place,  the 
amount  of  radium  chloride  that  could  be  obtained,  which 
was  of  sufficient  purity  for  atomic  weight  determinations, 
was  necessarily  small.  Indeed,  the  total  amount  of  chloride 
at  the  disposal  of  Mme.  Curie  was  only  about  one  hundred 
milligrams.  This  tended  to  magnify  all  experimental 
errors. 

The  chloride  of  radium,  which  is  hygroscopic,  shown  by 
the  fact  that  it  absorbs  water  when  in  a  desiccator  over 
drying  agents,  was  weighed  in  a  platinum  crucible.  Further, 
it  is  not  clear  that  any  test  was  made  to  determine  whether 
the  crystallized  radium  chloride  did  not  lose  hydrochloric 
acid  when  the  water  of  crystallization  was  removed.  It 
will  be  recalled  that  other  members  of  the  barium  group 
form  oxychlorides,  when  the  chlorides  are  dehydrated  in 


62         THE  ELECTRICAL  NATURE  OF  MATTER 

the  air.  It  is  well  known  that  the  chloride  of  calcium  can 
be  dehydrated  without  the  formation  of  oxy chloride,  only 
in  a  current  of  hydrochloric  acid  or  by  heating  with  ammo- 
nium chloride.  This  is  a  matter  that  should  certainly 
receive  attention  in  connection  with  the  method  of  deter- 
mining the  atomic  weight  of  radium,  that  was  employed 
by  Mme.  Curie. 

A  further  question  that  naturally  suggests  itself  in  con- 
nection with  the  method  is  this:  Does  silver  nitrate  pre- 
cipitate all  of  the  chlorine  from  radium  chloride  as  silver 
chloride?  The  properties  of  radium  are  so  remarkable, 
as  we  shall  learn,  that  it  does  not  follow  that  this  would 
necessarily  be  the  case. 

The  most  recent  determinations  of  the  atomic  weight 
of  radium  by  Mme.  Curie  l  and  by  Thorpe  2  give  values 
between  226  and  227,  and  it  must  be  said  that  these  pieces 
of  work  seem  to  have  been  carried  out  very  carefully. 

A  recalculation 3  recently  made  from  spectroscopic  data 
gives  essentially  the  same  value. 

Gray  and  Ramsay 4  found  the  atomic  weight  of  radium 
to  be  226.36.  Honingschmid  5  found  225.93. 

1  Comp.  rend.,  145,  422  (1907). 
2Ztschr.  anorg.  Chem.,  58,  443  (1908). 

3  Watts:  Phil.  Mag.  18,  411  (1909). 

4  Proc.  Roy.  Soc.;  86,  A,  270  (1912). 
6  Monatsh.  Chem.;  33,  253  (1912). 


CHAPTER  VII 
OTHER  RADIOACTIVE  SUBSTANCES  IN  PITCHBLENDE 

POLONIUM 

THERE  are  apparently  other  radioactive  substances  in 
pitchblende,  in  addition  to  radium,  as  we  have  seen.  There 
seems  to  be  a  new  radioactive  substance  in  this  mineral 
that  is  closely  allied  to  bismuth.  It  has  already  been  re- 
ferred to  under  the  name  of  polonium.1  It  is  precipitated 
along  with  the  bismuth,  from  the  hydrochloric  acid  solu- 
tion of  the  pitchblende  residue,  by  means  of  hydrogen  sul- 
phide. It  has  thus  far  been  impossible  to  free  the  supposed 
polonium  from  bismuth.  Partial  separation  has  apparently 
been  effected,  or,  at  least,  a  strongly  radioactive  substance 
has  been  obtained  by  precipitating  the  nitric  acid  solution 
by  water.  The  subnitrate  that  is  thrown  down  is  much 
more  radioactive  than  the  unprecipitated  portion. 

It  seems  yet  to  be  a  question  whether  this  radioactive 
bismuth  really  contains  a  new  radioactive  element,  or  is 
simply  bismuth  made  radioactive  by  the  deposition  upon 
it  of  a  substance  coming,  as  we  shall  learn,  from  the  radium 
in  the  pitchblende.  If  there  is  a  new  radioactive  element 
associated  with  the  bismuth,  it  might  reasonably  be  ex- 
pected to  show  definite  and  characteristic  lines  in  the  spec- 
trum, as  radium  does.  Demarcay,  who  worked  out  the 
spectrum  of  radium,  was  unable  to  find  any  new  lines  pro- 
duced by  the  radioactive  bismuth.  Sir  William  Crookes, 

1  Ann.  Chim.  Phys.,  30,  119  (1903). 
63 


64        THE  ELECTRICAL  NATURE  OF  MATTER 

on  the  other  hand,  announces  a  new  line  for  this  substance 
in  the  ultraviolet. 

If,  however,  it  should  be  shown  that  the  radioactive  bis- 
muth contains  no  new  line,  it  does  not  prove,  as  Mme. 
Curie  points  out,  that  there  is  no  new  element  contained  in 
this  substance,  since  there  are  many  elements  known  that 
do  not  have  any  well-characterized  spectrum.  An  experi- 
ment performed  by  Marckwald1  in  1902  may  throw  some 
light  on  the  nature  of  polonium.  If  a  stick  of  bismuth  is 
plunged  into  the  solution  of  active  bismuth  chloride  ob- 
tained from  pitchblende,  it  becomes  covered  with  a  black 
coating  which  is  extremely  radioactive,  and  the  remaining 
solution  is  no  longer  radioactive.  This  deposit  is  mainly 
tellurium,  with  a  very  small  amount  of  the  radioactive 
substances.  An  active  deposit  is  obtained  if  tin  chloride 
is  added  to  the  radioactive  bismuth  chloride.  Marckwald 
thinks  that  this  radioactive  element  is  analogous  to  tellu- 
rium, and  calls  it  radiotellurium.  It  has  properties  strik- 
ingly analogous  to  the  polonium  of  the  Curies,  the  analogy 
being  especially  marked  between  the  kinds  of  radiations 
sent  out  by  it.  More  work  is  required  to  show  whether 
these  substances  are  identical,  or  are  different. 

It  should,  however,  be  stated  that  the  fact  that  polonium 
is  precipitated  from  a  solution  of  radioactive  bismuth  by 
simply  introducing  a  piece  of  bismuth  would  alone  indicate 
that  these  substances  are  fundamentally  different.  It  is  well 
known  that  a  metal  cannot  precipitate  more  of  the  same 
metal  from  a  solution  of  any  of  its  salts.  In  order  that  a 
metal  may  be  able  to  precipitate  another  from  its  salts,  it 
is  necessary  that  the  metal  which  is  thrown  out  of  solution 
should  have  a  much  lower  solution-tension,  or  stand  lower 
in  the  tension  series,  than  the  metal  which  throws  it  out 

1  Ber.  d.  deutsch.  chem.  Gesell.,  35,  2285  (1902). 


OTHER   RADIOACTIVE    SUBSTANCES    IN   PITCHBLENDE       65 

and  takes  its  place.  The  metal  which  passes  into  solution 
must  have  the  power  to  take  the  charge  from  the  ion  of 
the  metal  that  is  thrown  out,  becoming  itself  an  ion,  while 
the  original  ion  is  converted  into  an  atom. 

/ 

ACTINIUM 

It  has  already  been  mentioned  that  Debierne  1  obtained 
from  pitchblende  an  active  substance,  which  is  termed 
actinium.  This  substance  is  quite  different  from  radium, 
and  also  from  polonium.  It  comes  out  of  pitchblende 
along  with  the  rare  earths,  and  especially  with  thorium,  to 
which  it  is  very  closely  allied.  This  is  probably  the  same 
substance  as  that  obtained  from  pitchblende  by  Giesel 
along  with  other  rare  elements  of  the  cerium  group.  Giesel 2 
called  the  substance  emanium  on  account  of  its  great 
emanating  power,  but  afterwards  found  that  it  was 
identical  with  the  actinium  of  Debierne.  He  found  that 
actinium  was  also  closely  allied  in  its  properties  to  lantha- 
num. It  could  be  partially  separated  from  the  lanthanum 
by  fractional  crystallization  of  the  double  nitrate  with 
manganese.  The  occurrence  of  actinium  with  thorium  has 
raised  the  question  as  to  whether  the  apparent  activity 
of  thorium  itself  is  not  really  due  to  the  admixture  of  a 
small  amount  of  actinium.  This  question  can  be  settled, 
as  Rutherford  points  out,  after  thorium  has  been  obtained 
which  is  devoid  of  radioactivity.  Since,  however,  it  is 
doubtful  whether  this  has  been  done,  it  would  be  premature 
to  conclude  that  the  radioactivity  of  thorium  was  due  to  the 
presence  of  small  amounts  of  actinium.  Indeed,  we  shall  see 
later  that  it  is  doubtful  whether  this  is  the  case  —  the  activity 

1  Compt.  rend.  (1899),  130,  906  (1900).     Also  (1903),  (1904),  (1905). 

2  Ber.  d.  chem.  Gesell.,  35,  3608  (1902):  36,  342  (1903);  37,  1696,  3963, 
(1904);  38,  775  (1905);  40,  3011  (1907). 


66         THE  ELECTRICAL  NATURE  OF  MATTER 

of  thorium  probably  being  due  to  the  presence  of  radiotho- 
rium.  No  spectrum  has  as  yet  been  observed  for  actinium. 

Other  radioactive  substances  have  been  announced  as 
coming  from  pitchblende.  It  is  probable  that  these  sub- 
stances either  contain  small  amounts  of  the  other  radio- 
active substances  known  to  exist  in  pitchblende,  such  as 
radium,  and  probably  polonium  and  actinium;  or  are  made 
radioactive  by  the  presence  of  other  radioactive  substances. 
We  shall  learn  that  certain  radioactive  substances  have  the 
property  of  making  other  substances  in  contact  with  them 
radioactive. 

This  kind  of  radioactivity  is  known  as  induced  radio- 
activity. We  shall  become  more  familiar  with  this  subject 
when  we  come  to  study  more  closely,  in  a  subsequent  chap- 
ter, the  nature  of  the  radiations  given  off  by  radioactive 
substances. 

We  have  now  taken  a  brief  survey  of  the  steps  involved 
in  the  discovery  and  isolation  of  the  radioactive  elements, 
and  especially  of  the  best  known  of  them  all  —  radium. 
The  next  step  in  order  of  logical  sequence  is  to  study  the 
properties  of  these  various  substances,  starting,  perhaps, 
with  the  less  active,  uranium  and  thorium,  and  then  taking 
up  the  more  active,  especially  radium,  about  which  so  much 
and  such  important  knowledge  has  already  been  gained. 

The  methods  that  have  been  employed  in  these  inves- 
tigations are  not  obvious,  and,  therefore,  should  be  briefly 
considered  before  the  results  that  have  been  obtained 
through  their  application. 

THE  MORE  IMPORTANT  METHODS  USED  IN  STUDYING  RADIO- 
ACTIVITY 

The  methods  that  have  been  employed  in  studying  radio- 
activity are  based,  of  course,  upon  the  properties  of  the 


OTHER   RADIOACTIVE   SUBSTANCES   IN  PITCHBLENDE        67 

radiations  that  are  given  out  by  the  various  radioactive 
substances. 

We  have  seen  that  such  substances  affect  a  photographic 
plate  exposed  to  their  radiations.  It  will  be  remembered 
that  it  was  by  means  of  this  property  that  Becquerel  discov- 
ered the  first  radioactive  substance  —  uranium.  Although 
this  method  is  still  used  for  certain  purposes,  there  are  a 
number  of  objections  to  its  general  use  in  connection  with  the 
study  of  radioactivity.  In  the  first  place,  it  is  not  sufficiently 
sensitive  for  work  with  weakly  radioactive  substances. 

Another  serious  objection  to  the  photographic  method 
is  that  certain  radiations  given  off  from  radioactive  sub- 
stances, even  when  fairly  intense,  have  very  slight  action 
upon  the  photographic  plate.  Another  objection  to  the 
photographic  method  is  a  somewhat  general  one.  Photo- 
graphic plates  are  sensitive  to  such  a  number  of  agents. 
Many  things  when  brought  in  contact  with  a  photographic 
plate  leave  an  imprint  on  the  plate  when  it  is  developed. 
This  can,  however,  be  overcome  by  suitable  precautions, 
and  photography  has  proved  of  invaluable  service  in  the 
development  of  scientific  knowledge. 

Taking  all  of  these  facts  into  account,  the  photographic 
method  is  not  well  adapted  to  the  study  of  radioactivity 
in  general,  although  it  has  certain  special  applications 
that  are  important. 

Another  property  of  radioactive  substances  is  to  cause 
certain  substances  upon  which  their  radiations  fall,  to 
phosphoresce.  This  is  especially  true  if  the  radiations 
are  allowed  to  fall  upon  screens  covered  with  the  beautiful 
salt  barium  platinocyanide.  The  fluoroscopic  method  is 
of  very  limited  applicability,  since  weakly  radioactive  sub- 
stances do  not  produce  enough  phosphorescence  in  these 
screens  to  be  observed. 


68        THE  ELECTRICAL  NATURE  OF  MATTER 

We  have  already  seen  that  the  radiations  from  radioactive 
substances  have  the  power  to  discharge  charged  bodies 
surrounded  by  a  gas  such  as  the  atmosphere.  This  means 
that  such  radiations  have  the  power  to  render  a  gas  like 
the  air  a  conductor  of  electricity.  In  a  word,  to  ionize 
the  gas  into  the  negative  electron  and  the  relatively  large 
positive  ion. 

A  method  based  upon  this  property  of  the  radiations  has 
proved  of  the  greatest  service  in  connection  with  the  study 
of  radioactivity.  Indeed,  it  is  the  only  method  that  is 
capable  of  giving  reliable  quantitative  measurements. 

For  details  concerning  the  measurements  of  the  conduc- 
tivities of  gases  through  which  the  radiations  from  radio- 
active substances  are  passing,  the  original  investigations, 
especially  of  Rutherford,  must  be  consulted. 

PROPERTIES    OF    THE    RADIATIONS    GIVEN    OUT    BY    RADIO- 
ACTIVE  SUBSTANCES 

We  have  already  become  familiar  with  the  fact  that 
radioactive  substances  give  out  radiations  that  have  the 
property  of  affecting  a  photographic  plate,  of  rendering 
certain  substances  phosphorescent,  and  of  ionizing  gases. 

The  question  would  naturally  be  raised,  are  the  radia- 
tions given  out  by  all  radioactive  substances  the  same  in 
character?  Again,  are  all  the  radiations  given  out  by  any 
one  radioactive  substance  of  the  same  nature? 

These  questions  are  easily  asked,  but  can  be  answered 
only  by  experimental  work,  and  this  not  always  of  a  very 
simple  kind.  It  is,  however,  not  a  difficult  matter  to  show 
qualitatively  that  the  radiations  given  out  by  a  radioactive 
substance,  such  as  radium,  are  not  homogeneous,  but  are 
complex  in  character. 

If  we  charge  a  gold-leaf  electroscope  and  subject  it  to 


OTHER    RADIOACTIVE    SUBSTANCES    IN    PITCHBLENDE       69 

the  radiation  from  radium,  it  will  be  rapidly  discharged, 
due  to  the  ionization  of  the  air  produced  by  these  radiations. 
If  now  we  interpose  between  the  radium  salt  and  the  elec- 
troscope a  thin  sheet  of  metal,  or  even  a  piece  of  paper, 
the  electroscope  will  be  discharged  much  more  slowly, 
showing  that  a  portion  of  the  radiation  has  been  cut  off. 
If  we  then  interpose  into  the  path  of  the  rays  a  thick  piece 
of  metal,  the  electroscope  will  be  discharged  much  more 
slowly  than  when  a  piece  of  metal  foil  was  used,  and  the 
difference  will  not  be  proportional  to  the  thickness  of  the 
piece  of  metal  introduced.  The  interposition  of  a  second 
such  piece  of  metal  has  but  little  effect. 

These  qualitative  experiments  show  conclusively  that  the 
radiation  from  radium  is  heterogeneous,  consisting  of  dif- 
ferent kinds  of  rays.  The  most  natural  interpretation  of 
these  results  would  be  that  the  piece  of  thin  sheet  metal, 
or  metal  foil,  cuts  off  a  kind  of  radiation  that  has  relatively 
little  power  to  penetrate  matter;  and  that  the  thick  piece  of 
metal  cuts  out  a  more  penetrating  kind  of  radiation,  letting 
a  third,  highly  penetrating  form  pass  through,  which  of 
itself  is  capable  of  ionizing  the  gas  to  a  slight  extent  and 
slowly  discharging  the  electroscope. 

While  this  is,  perhaps,  the  most  obvious  interpretation 
of  the  results  of  the  above  described  experiment,  it  remains 
to  be  seen  whether  it  is  the  correct  one. 

Giesel l  took  up  the  study  of  the  effect  of  the  magnetic 
field  on  the  radiations  from  radium  in  general.  Results  of 
the  very  highest  importance  were  obtained.  He  found  that 
at  least  some  of  the  radiations  from  radium  could  be  de- 
flected by  the  magnetic  field,  which  accounted  for  the  change 
in  the  conductivity  produced  in  the  air  by  the  radiations 
when  these  were  made  to  pass  through  a  magnetic  field. 
1  Wied.  Ann.,  68,  834  (1899). 


70        THE  ELECTRICAL  NATURE  OF  MATTER 

A  little  later  M.  Curie  l  showed  that  the  radiations  from 
radium  consisted  of  two  kinds,  one  that  was  not  deflected 
or  deviated  in  the  magnetic  field,  and  another  that  was 
deviated  by  the  field.  The  kind  that  was  not  deviated 
had  very  little  penetrating  power,  and  was  the  kind  that  is 
so  readily  stopped  even  by  a  thin  sheet  of  metal  foil. 

The  kind  that  was  deviated  by  the  magnetic  field  had 
much  greater  penetrating  power,  and  was  capable  of  pass- 
ing through  thin  sheets  of  metal.  It  could  not,  however, 
pass  through  sheets  of  metal  of  any  appreciable  thickness. 

About  the  same  time  it  was  shown  by  Villard  2  that  the 
radiations  from  radium  contain  a  third  kind  of  rays,  that 
have  very  great  penetrating  power,  and  are  not  deviable 
by  the  magnetic  field.    The  radiations  from  radium  con- 
tain, then,  three  kinds  of  rays,  each  with  its  own  definite, 
characteristic  properties.     These  have  been  named  the 
Alpha  (a)  rays. 
Beta  ()8)  rays. 
Gamma  (y)  rays. 

A  fourth  kind  of  radiation,  the  S  rays,  will  also  be  considered. 

We  can  now  understand  the  qualitative  experiment  dis- 
cussed earlier  in  this  chapter. 

The  thin  sheet  of  metal  cut  off  the  a  radiations,  but 
allowed  most  of  the  /3,  and  practically  all  of  the  y  radia- 
tions to  pass  through.  When  the  a  rays  were  cut  off  the 
air  was  ionized  much  less  rapidly,  for,  as  we  shall  learn, 
the  a  rays  are  the  chief  ionizing  agents  in  the  radium  radia- 
tions, and  the  electroscope  was  discharged  much  less  rapidly 
than  when  they  were  allowed  to  pass  through  the  air 
between  the  leaves  of  the  electroscope. 

The  thick  piece  of  metal  cut  off  the  /3  radiations  and 

1  Compt.  rend.,  130,  73  (1900). 

2  Ibid.,  130,  1178  (1900). 


OTHER   RADIOACTIVE    SUBSTANCES    IN   PITCHBLENDE       71 

allowed  only  the  y  radiations  to  pass.  The  electroscope 
was  now  discharged  much  more  slowly,  since  the  y  radia- 
tions have  less  power  to  ionize  a  gas  than  even  the  ft  radia- 
tions, which  in  turn  have  much  less  ionizing  power  than 
the  a  radiations. 

Our  original  conclusion  from  the  facts  of  the  qualitative 
experiment  is  then  correct.  The  radiations  from  radium 
consist  chiefly  of  three  distinct  kinds  of  rays;  the  fourth  or 
8  rays  having  been  discovered  comparatively  recently. 

We  shall  now  proceed  to  study  the  properties  of  these  in 
some  detail,  taking  them  up  in  the  order,  alpha,  beta,  and 
gamma,  and  not  in  the  order  of  their  discovery. 

It  may  be  said  in  advance  that  all  three  radioactive  sub- 
stances, uranium,  thorium,  and  radium,  give  out  these 
three  types  of  radiations.  Polonium,  as  we  shall  learn, 
gives  out  only  one  type,  the  a  radiations. 


CHAPTER  VIII 
THE  ALPHA  RAYS 

IT  has  already  been  mentioned  that  the  a  rays  are  only 
slightly  deviable  in  a  magnetic  field,  that  they  have  very 
little  power  to  penetrate  matter,  and  that  they  produce 
most  of  the  ionization  of  the  gas  through  which  the  radia- 
tions from  radium  pass. 

The  study  of  the  deviation  of  the  a  rays  in  a  magnetic 
field  we  owe  largely  to  Rutherford.1  That  they  are  deviated 
was  shown  by  the  following  simple  experiment.  If  some 
radium  salt  is  placed  in  the  bottom  of  a  narrow  tube,  which 
in  turn  is  introduced  between  the  poles  of  an  electro-magnet, 
radiations  from  the  salt  will  fall  upon  an  electroscope  placed 
directly  in  front  of  the  tube.  If  the  current  is  now  turned 
on  the  electromagnet,  any  rays  that  are  appreciably  de- 
flected by  the  magnet  would  fall  upon  the  side  walls  of  the 
tube,  and  would  not  reach  the  electroscope. 

The  number  of  experimental  difficulties  that  had  to  be 
overcome  was  large.  The  tube  or  slit  in  which  the  salt  was 
placed  must  be  small,  in  order  that  the  rays  might  be  bent 
enough  to  strike  the  walls.  To  augment  the  effect  a  num- 
ber of  such  slits  were  used. 

After  all  of  the  experimental  difficulties  had  been  over- 
come, Rutherford  showed  that  when  a  powerful  magnetic 
field  was  used,  all  of  the  a  rays  were  deviated.  This  proved 
that  the  a  rays  are  made  up  of  charged  particles.  It  does 

*  Phil.  Mag.,  5, 177  (1903). 
72 


THE  ALPHA   RAYS  73 

not,  however,  show  whether  the  particles  are  charged  posi- 
tively or  negatively.  If  the  particles  are  charged  positively 
the  rays  would  be  deviated  in  one  direction,  if  negatively 
in  the  opposite  direction.  It  was  found  that  the  a  rays  are 
deviated  in  a  direction  which  is  exactly  opposite  to  that  in 
which  another  class  of  rays,  known,  as  we  shall  see,  to  con- 
sist of  negatively  charged  particles,  is  deviated.  This  proves 
that  the  a  rays,  at  least  from  radium,  are  composed  of  posi- 
tively charged  particles.  The  presence  of  a  positive  charge 
upon  the  a  particles  was  demonstrated  directly  by  J.  J. 
Thomson.1  He  used  a  radioactive  substance  which  gives 
off  only  a  rays.  Some  of  this  substance  was  placed  at  a 
distance  of  three  centimetres  from  a  metal  plate  which 
was  connected  with  a  gold-leaf  electroscope.  When  a 
vacuum  was  established  the  electroscope  leaked  very 
rapidly  if  positively  charged,  but  only  very  slowly  if  neg- 
atively charged.  When  the  apparatus  was  placed  in  a 
strong  magnetic  field  the  positive  leak  was  slight,  due 
to  the  electrons  being  bent  away  by  the  field.  The  experi- 
ment was  then  tried  of  placing  the  radiotellurium  closer 
to  the  metal  plate  in  a  strong  magnetic  field.  Under  these 
conditions  the  electroscope  became  charged  positively, 
showing  that  the  a  particles  were  charged  positively. 
Recent-experiments  by  Rutherford  led  to  exactly  the  same 
result. 

It  will  be  remembered  that  the  a  rays  are  given  off  from 
all  radioactive  substances,  and,  further,  that  only  a  and  8 
rays  are  given  off  from  polonium.  A  question  that  should  be 
raised  and  answered  is  this,  are  the  a  rays  from  polonium 
the  same  in  character  as  the  a  rays  from  other  radioactive 
substances?  This  was  tested  by  Becquerel  in  1903.  He 
showed  that  the  a  rays  from  polonium  are  deflected  in  the 

1  Phil.  Mag.,  10,  193  (1905). 


74        THE  ELECTRICAL  NATURE  OF  MATTER 

magnetic  field  in  the  same  direction  as  the  a  rays  from 
radium.  The  a  rays  from  polonium,  therefore,  consist 
also  of  positively  charged  particles. 

The  conclusion  that  the  a  rays  consist  of  electrically 
charged  particles  was  confirmed  by  Rutherford  in  the 
following  manner.  The  rays  were  passed  through  an  elec- 
tric field,  and  were  shown  to  be  deviated  by  the  field.  The 
a  particles  are  charged;  each  particle  carrying  two  unit 
positive  charges. 


e 

THE  RATIO  —  FOR  THE  ALPHA  PARTICLE 

m 

The  ratio  of  the  charge  to  the  mass  of  the  a  particles 
can  be  ascertained  by  the  same  general  method  as  that 
which  was  employed  by  J.  J.  Thomson  for  determining 
the  same  ratio  for  the  cathode  particle.  This  has  already 
been  discussed  at  some  length  in  an  earlier  chapter.  By 
studying  the  deviation  of  the  rays  in  both  a  magnetic  and 
electrostatic  field,  as  we  have  seen,  it  is  possible  to  deter- 

/> 

mine  the  velocity  of  the  particles  and  the  ratio  — . 

m 

Very  different  results  were  obtained  with  the  a  particles 
from  those  reached  by  Thomson  for  the  cathode  particles. 
The  mean  velocity  of  the  a  particles  is  about  2.5  X  io9  cen- 
timetres per  second,  which  is  about  one-tenth  the  velocity 
of  light.  The  ratio  of  charge  to  mass  for  the  a  particle  is 
about  6Xio3.  While  this  result  must  riot  be  regarded  as 
very  accurate,  on  account  of  the  difficulty  in  obtaining  a 
large  deviation  in  the  electrostatic  field,  it  is  still  of  the 
right  order  of  magnitude. 

It  is  interesting  to  compare  this  result  with  that  found 
for  the  cathode  particle. 


THE   ALPHA  RAYS  75 

The  velocity  of  the  cathode  particle  is  about  3Xio9 

centimetres  per  second,  and  the  ratio  —  =  io7. 

m 

The  cathode  particle,  therefore,  moves  faster  than  the  a 
particle,  and  has  a  value  of  — ,  which  is  about  two  thousand 

times  as  great  as  that  of  the  a  particle. 

Rutherford  l  has  recently  shown  that  the  a  rays  from 
radium  are  complex,  consisting  of  particles  projected  at 
different  velocities.  It  will  be  seen  on  page  70  that  there 
are  four  different  products  produced  by  radium,  and 
radium  itself,  which  gives  off  a  particles.  The  a  rays  from 
radium  c  pass  through  about  twice  the  thickness  of  air 
that  the  a  rays  from  radium  itself  do.  Thus,  each  pro- 
duct from  radium  seems  to  give  off  a  particles  at  a  certain 
definite  velocity.  To  measure  the  velocity  of  the  a  particles, 
those  emitted  by  only  one  product  must  be  studied  at  a 
time.  Bragg  and  Kleeman  2  have,  however,  shown  that 
the  a  particles  given  off  by  radium  in  any  one  stage  of  its 
decomposition  are  of  the  same  nature. 


THE  MASS  OF  THE  APLHA  PARTICLE 

Knowing  the  value  of  — ,  we  have  become  familiar  with 
m 

a  method  worked  out  by  J.  J.  Thomson  for  determining 
the  value  of  e  and,  therefore,  the  value  of  m.  While  these 
determinations  have  not  been  carried  out  directly  for  the  a 
particles  as  for  the  cathode  particle,  still  some  light  has  been 

thrown  on  the  present  problem.    .We  have  seen  that  the 

& 
ratio  —  for  the  a  particle  is  about  6  X  io3. 

m 

1Phil.  Mag.,  io,  163  (1905). 
*Ibid.}  io,  318  (1905). 


76       THE  ELECTRICAL  NATURE  OF  MATTER 

The  ratio  of  —  for  the  hydrogen  ion  in  the  solution  of 
tn 

acids  is,  as  we  have  seen,  about  io4. 

If  the  charge  carried  by  the  a  particle  is  twice  that 
carried  by  the  hydrogen  ion  in  solution,  as  is  made  highly 
probable  by  our  general  knowledge  of  these  bodies,  then 
we  can  compare  the  masses  of  the  hydrogen  ion  and  of  the 

c  e 

a  particle.     Since  —  for  the  former  is  io4,  and  —  for  the 

latter  6  X  io3,  it  follows  that  the  mass  of  the  a  particle  is 
about  four  times  the  mass  of  the  hydrogen  ion.  It  will  be 

e 
recalled  that  the  determination  of  —  for  the  a  particle  is 

only  approximate.  It  is  therefore  possible  that  the  mass 
of  the  a  particle  is  just  four  times  as  great  as  that  of  the 
hydrogen  ion,  in  which  case  it  would  be  equal  to  the  mass 
of  the  helium  atom.  We  shall  see  that  there  is  strong 
evidence  in  favor  of  the  view  that  the  a  particles  are  charged 
helium  atoms. 

We  have  seen  that  the  a  particles  are  projected  with 
enormous  velocities,  2.5Xio9  centimetres  per  second.  If 
they  have  masses  even  as  great  as  the  hydrogen  atoms  or 
ions,  with  such  velocities  they  would  have  a  large  amount 
of  energy.  This  is  the  probable  explanation  of  their  great 
power  to  ionize  a  gas  through  which  they  pass,  and  to  pro- 
duce other  effects  with  which  we  shall  become  familiar 
somewhat  later. 

The  recent  work  of  Mackenzie  l  carried  out  in  the  lab- 
oratory of  J.  J.  Thomson  has  given  values  somewhat 
different  from  the  above.  He  used  the  magnetic  and  elec- 
trostatic deflection  of  the  a  particles,  and  by  this  means  the 
velocity  of  the  particles  could  be  determined,  and  also  the 

iphil.  Mag.,  io,  538  (1905). 


THE   ALPHA   RAYS  77 

ratio  of  the  charge  e  to  the  mass  m.  In  this  work,  radium 
which  was  in  radioactive  equilibrium  was  employed. 
Under  these  conditions  radium  is  sending  out  a  particles 
with  very  different  velocities,  and  what  is  really  determined 
is  the  mean  velocity. 

In  the  magnetic  deflection  of  the  rays  the  a  particles  from 
the  radium  entered  a  brass  vacuum-box  by  passing  through 
a  thin  sheet  of  mica.  The  rays  passed  through  a  vacuum  for 
about  fifteen  centimetres,  and  then  fell  on  a  screen  of  zinc 
sulphide.  The  line  of  scintillations  was  then  photographed. 

The  poles  of  an  electromagnet  could  be  placed  along  the 
path  of  the  rays,  and  when  the  magnetic  field  was  applied 
the  usual  deflection  of  the  a  particles  took  place.  This  was 
registered  photographically. 

The  mean  value  found  for  -  -  was  3.ooXio5,  varying 

c 

between  the  extremes  2.5  X  io5  and  3.7  X  io5. 

The  value  of   '   -  found  by  Rutherford  for  radium  in 

€ 

radioactive  equilibrium  is  3.9Xio5.  The  Mackenzie  value 
must  be  corrected  for  the  decrease  in  the  velocity  of  the 
particles  produced  by  passing  through  the  thin  sheet  of 
mica.  The  corrected  values  are  as  follows:  The  average 

value  of  -  -  for  the  a  particles  as  they  leave  the  surface 
& 

of  the  radium  is  3.i8Xio5;  the  extreme  values  being  2.65  X 
io5  and  3.92Xio5. 

In  measuring  the  electrostatic  deflection  an  apparatus 
was  employed  which  was  similar  in  many  respects  to  that 
used  in  measuring  the  magnetic  deflection.  The  a  rays 
entered  the  apparatus  by  passing  through  the  mica  plate, 
but  they  were  now  passed  between  two  plates  charged  to  a 
difference  of  potential  as  great  as  10,000  volts. 


78        THE  ELECTRICAL  NATURE  OF  MATTER 

The  value  found  for  —  =  4.nXio14. 
c 

The  value  of  — -  =  3-ooXio5. 
c 

The   average   value   of   v  =  1.37X1  o9   centimetres  per 

p 

second,  and  —  =  4.6  Xio3  electromagnetic  units. 
m 

The  magnetic  deflection  of  the  a  particles  from  polo- 
nium was  also  measured,  and  these  were  found  to  have 
somewhat  greater  velocity  than  the  average  a  particles 
from  radium. 

Their  velocity,  however,  was  not  as  great  as  the  swift- 
est a  particles  from  radium. 


THE   SPINTHARISCOPE 

Another  matter  must  be  discussed  before  leaving  the 
a  rays.  It  has  already  been  stated  that  strongly  radio- 
active substances  like  radium  can  produce  phosphorescence 
in  certain  substances  exposed  to  their  radiations.  Thus, 
screens  covered  with  barium  platinocyanide  or  zinc  sul- 
phide become  phosphorescent  when  exposed  to  the  action 
of  radium  radiations. 

This  power  of  the  radiations  from  radium  to  produce 
phosphorescence  can  readily  be  shown  to  be  due  mainly 
to  the  a  rays.  If  the  a  rays  are  cut  off  by  a  thin  screen 
of  metal,  most  of  the  power  of  the  radiations  to  produce 
phosphorescence  is  lost. 

The  power  of  the  a  particles  to  produce  phosphorescence 
has  been  utilized  by  Sir  William  Crookes  l  in  the  following 
manner.  If  a  plate  covered  with  phosphorescent  zinc 
sulphide  is  exposed  to  the  radiations  from  radium  or  polo- 
nium, at  a  short  distance  from  the  substance,  it  presents 
1  Roy.  Soc.  Proceed.,  71,  405  (1903).  Chem.  News,  87,  (1903). 


THE   ALPHA   RAYS  79 

a  remarkable  appearance.  The  screen  does  not  become 
homogeneously  phosphorescent  throughout,  but  bright 
points  of  light  make  their  appearance,  and  rapidly  dis- 
appear. The  best  result  is  obtained  by  examining  the  screen 
through  a  small  lens.  Based  upon  these  facts  is  Crookes' 
spinthariscope.  At  one  end  of  a  tube  is  placed  a  piece  of 
metal  which  contains  some  radium  chloride  or  bromide  on 
its  surface.  This  is  suspended  at  a  distance  of  a  few  milli- 
metres from  a  screen  covered  with  phosphorescent  zinc 
sulphide.  The  other  end  of  the  tube  contains  a  magnify- 
ing lens.  This  instrument  has  been  termed  a  spinthari- 
scope, from  "  spintharis"  a  spark. 

The  appearance  of  the  screen  has  been  described  as  analo- 
gous to  that  of  the  milky  way  as  seen  with  the  naked  eye 
on  a  dark  night.  Bright  points  of  light  appear  and  quickly 
disappear  all  over  the  screen.  These  come  and  go  in  rapid 
succession.  The  effect  has  also  been  described  as  analo- 
gous to  the  splashing  of  drops  of  rain  in  a  pool. 

The  cause  of  this  remarkable  phenomenon  is  probably 
the  impact  of  the  a  particles  upon  the  screen  covered  with 
the  substance  in  which  phosphorescence  can  be  set  up. 
The  a  particles,  on  account  of  their  high  velocity  and  appre- 
ciable mass,  have,  as  we  have  seen,  a  considerable  amount 
of  kinetic  energy.  When  they  fall  upon  the  screen  covered 
with  zinc  sulphide,  they  are  stopped,  and  produce  a  me- 
chanical disturbance.  Zinc  sulphide  becomes  luminous 
when  subjected  to  almost  any  mechanical  disturbance. 
Merely  rubbing  it  with  a  hard  surface  will  render  it  phos- 
phorescent. Wherever  an  a  particle  falls  upon  the  screen, 
that  portion  of  the  screen  becomes  luminous  for  some  dis- 
tance around  the  point  of  collision.  Every  spark  or  centre 
of  luminous  disturbance  on  the  screen  is  the  result  of  the 
impact  of  an  a  particle  upon  the  screen.  We  thus  see,  as 


80        THE  ELECTRICAL  NATURE  OF  MATTER 

it  were,  the  points  at  which  the  separate  a  particles  strike 
the  phosphorescent  screen,  and  this  is,  perhaps,  one  of  the 
best  examples  of  the  action  of  individual  atoms  or  molecules 
made  directly  perceptible  to  any  of  our  senses. 

Another  theory  of  the  action  of  the  spinthariscope  has 
been  proposed  by  Becquerel.1  He  thinks  the  scintillation  is 
due  to  a  fracture  of  the  crystals  of  the  phosphorescent  zinc 
sulphide  by  the  a  particles.  He  does  not  think  that  such 
a  fracture  could  be  produced  by  one  a  particle,  but  only 
when  a  number  of  such  particles  strike  simultaneously  a 
weak  point  in  the  crystal.  That  light  is  frequently  emitted 
when  crystals  are  crushed,  is  well  known.  Indeed,  crystals 
of  zinc  sulphide  give  out  light  when  mechanically  crushed, 
and  according  to  Becquerel  such  light  has  the  characteris- 
tics of  that  in  the  spinthariscope.  It  shows  the  same  gen- 
eral kind  of  scintillations,  and  the  number  of  scintillations 
is  dependent  somewhat  upon  the  size  of  the  crystals  of 
zinc  sulphide  with  which  the  screen  is  covered.  The 
smaller  the  crystals  of  the  sulphide  the  larger  the  number 
of  scintillations,  which  accords  with  Becquerel's  view  as 
to  the  action  in  the  spinthariscope.  The  smaller  the  crystals 
the  more  easily  they  would  be  broken,  and,  consequently, 
the  larger  the  number  of  scintillations.  It  is  difficult  at 
present  to  decide  between  these  two  views.  The  theory 
first  advanced  is  the  simpler  and  more  fascinating,  but 
it  may  not  be  true.  More  experimental  evidence  must  be 
obtained  before  a  final  decision  can  be  reached. 

CRITICAL  VELOCITY   OF  THE  ALPHA  PARTICLES 

Rutherford  and  other  investigators  have  found  that  all 
of  the  a  particles  given  out  by  a  radioactive  substance 
have  the  same  velocity.  When  the  a  particles  pass  through 

1  Compt.  rend.,  137,  629  (1903). 


THE   ALPHA   RAYS  8 1 

matter  their  velocity  decreases.  When  their  velocity  falls 
below  a  certain  value,  0.82  X  io9  centimetres  per  second,  they 
cease  to  ionize  the  air,  and  they  no  longer  affect  the  photo- 
graphic plate  or  produce  phosphorescence  in  a  phosphores- 
cent screen.  This  is  known  as  the  critical  velocity  of  the  a 
particles.  We  have  no  means  of  detecting  the  presence  of 
a  particles  given  off  with  a  velocity  less  than  the  critical, 
and,  therefore,  cannot  determine  whether  such  exist  or  not. 
The  distance  which  the  a  particles  will  travel  in  air  is 
known  as  the  range  of  the  a  particles  —  a  term  very  fre- 
quently used. 

ALPHA  PARTICLES  PRODUCE  DELTA  PARTICLES 

We  have  just  seen  that  when  the  velocity  of  the  a  par- 
ticle falls  below  a  certain  value  it  ceases  to  ionize  a  gas 
through  which  it  passes.  Duane  1  thinks  that  the  a  par- 
ticle loses  its  charge  at  the  same  time  that  it  ceases  to 
ionize  the  surrounding  gas. 

When  a  particles  impinge  upon  a  solid  body,  8  particles 
are  produced  at  the  surface  of  the  solid.  Duane  shows  that 
beyond  their  "  range  "  the  a  particles  cease  to  produce 
8  particles. 

This  raises  the  question,  what  are  the  8  particles?  They 
are  particles  carrying  a  negative  charge  and  moving  with 
a  velocity  of  3.3Xio8  centimetres  per  second.  When  we 
come  to  study  in  some  detail  the  properties  of  the  ft 
particles  we  shall  see  that  the  8  particles  are  essentially 
nothing  but  slow-moving  ft  particles. 

ALPHA  PARTICLES  ARE  PROBABLY  HELIUM  ATOMS 

It  is  a  well-known  fact  that  helium  accumulates  in  radium, 
in  actinium  and  in  thorium  compounds.  This  has  led  to 

1  Amer.  Jour.  ScL,  26,  Nov.  (1908). 


82        THE  ELECTRICAL  NATURE  OF  MATTER 

the  suggestion  that  the  helium  consists  of  a  particles  that 
have  lost  their  charge.  This  is,  however,  difficult  to  test 
satisfactorily,  on  account  of  the  difficulty  of  measuring  e 
accurately.  Regener  1  has  devised  a  method  of  counting 
the  number  of  a  particles  under  certain  conditions,  by 
allowing  them  to  strike  a  screen  of  zinc  sulphide  and  pro- 
duce the  well-known  scintillations  —  each  a  particle  being 
supposed  to  produce  one  scintillation.  Knowing  the  num- 
ber of  a  particles  and  the  total  charge  carried  by  them,  we 
know  the  charge  carried  by  one  a  particle. 

The  same  result  has  been  obtained  by  Rutherford  and 
Geiger2  who  have  been  able  to  count  the  number  of  a  parti- 
cles given  off  by  uranium,  thorium,  radium,  and  actinium 
compounds,  by  increasing  the  ionizing  power  of  these  parti- 
cles in  accordance  with  a  principle  discovered  by  Townsend,3 
which  gives  the  conditions  under  which  ions  can  be  formed 
by  collision  between  neutral  gas  molecules  and  ions  moving 
in  a  strong  electric  field. 

Rutherford  and  Geiger4  thus  count  the  number  of  a 
particles  given  off  from  a  radioactive  substance  under 
given  conditions  and  allow  the  total  charge  that  they  carry 
to  accumulate  on  an  insulated  plate.  They  measure  the 
total  charge  carried,  and  knowing  the  number  of  carriers, 
they  know  the  charge  carried  by  one  a  particle.  This  was 
shown  to  be  9.3Xio~10  electrostatic  units,  which  gives  a 
mass  of  four  for  the  a  particle,  this  being  the  mass  of  the 
helium  atom. 

Dewar  5  measured  very  carefully  the  rate  at  which  helium 
is  produced  from  radium,  and  found  that  his  result  agreed 

1  Ber.  d.  physik.  Ges.,  10,  78  (1908). 

2  Proceed.  Roy.  Soc.,  A,  81,  141. 

a  Phil.  Mag.  5,  389,  698;  6,  358,  598  (1903). 
4  Proceed.  Roy.  Soc.,  A,  81,  141. 
6  Proceed.  Roy.  Soc.,  A,  81,  280. 


THE   ALPHA   RAYS  83 

very  well  with  that  calculated  on  the  assumption  that  the 
a  particles  are  charged  helium  atoms. 

Again,  Rutherford  and  Royds 2  showed  that  helium  can 
be  obtained  from  accumulated  a  particles,  independent  of 
the  active  matter  from  which  the  a  particles  came. 

Taking  all  of  the  above  facts  into  consideration,  the  evi- 
dence that  a  particles  are  simply  charged  helium  atoms 
is  very  striking. 

ACTION  OF  THE   tt  PARTICLES  ON  A  PHOTOGRAPHIC  AND  ON 
A  FLUORESCENT  PLATE 

When  the  a  particles  pass  through  matter,  their  velocity 
is  diminished.  When  their  velocity  falls  below  a  certain 
value  they  lose  their  properties  of  producing  luminescence, 
of  affecting  a  photographic  plate  and  of  ionizing  gases. 
The  important  point  is  that  this  value  is  the  same  in  all 
three  cases.  This  would  indicate,  as  Rutherford  points  out, 
that  the  three  properties  mentioned  above  have  a  common 
origin. 

The  absorption  of  the  a  rays  by  gases  is  due  to  the  energy 
being  used  up  in  producing  ions  in  the  gas.  Rutherford 
thinks  that  the  phosphorescent  action  and  the  action  on 
a  photographic  plate  are  primarily  the  action  of  ions. 
These  would  cease  at  about  the  same  velocity  that  would 
just  be  necessary  to  ionize  a  gas. 

The  bearing  of  these  results  on  the  action  of  the  spin- 
thariscope is  pointed  out.  Becquerel  explains  the  action,  as 
will  be  recalled,  as  due  to  the  cleavage  of  the  crystals  of 
the  phosphorescent  substance.  The  action  is  probably 
to  be  ascribed,  according  to  Rutherford,  to  the  production 
of  ions  in  the  substance.  When  these  ions  recombine  scin- 
tillations result. 

We  cannot  ascribe  the  action  of  this  instrument  simply 


84         THE  ELECTRICAL  NATURE  OF  MATTER 

to  the  bombardment  of  the  phosphorescent  screen  by  the 
a  particles,  since  we  have  just  seen  that  these  particles 
produce  no  scintillations  or  luminescence  after  their  ve- 
locity has  fallen  below  a  certain  definite  value,  and  they 
still  have,  of  course,  considerable  kinetic  energy. 

Rutherford  raises  the  question  as  to  whether  phospho- 
rescent and  photographic  effects  in  general  may  not  be  due 
primarily  to  the  production  of  ions. 

STOPPING  POWER  OF  MATTER  FOR  THE  ALPHA  PARTICLES 

The  following  interesting,  although  empirical  relations, 
have  apparently  been  established  by  Bragg  and  Kleeman. 
The  so-called  "  stopping  power  "  of  a  number  of  the  ele- 
ments for  the  a  particle  was  determined,  with  the  result 
that  the  amount  of  energy  spent  by  the  a  particle  in  producing 
ionization  in  an  atom  seems  to  be  proportional  to  the  square 
root  of  the  atomic  weight  of  the  substance  ionized.  Quite  a 
number  of  elements  have  been  brought  within  the  scope 
of  this  investigation,  with  the  result  that  the  above  relation 
seems  to  hold  approximately. 

They  have  also  shown  that  the  number  of  ions  produced 
by  an  a  particle  is  the  same,  no  matter  what  the  nature  of 
the  gas  through  which  it  passes;  and,  further,  that  the 
same  amount  of  energy  is  always  required  to  make  a  pair 
of  ions,  regardless  of  the  nature  of  the  atom  or  molecule 
from  which  they  came. 

This  latter  relation  is  probably  very  important,  since  it 
shows  that  ionization  is  essentially  the  same  process,  regard- 
less of  the  nature  of  the  molecules  of  the  gas  in  which  it 
takes  place. 


CHAPTER  IX 
THE  BETA  AND  GAMMA  RAYS 

THE  BETA  RAYS 

IT  was  pointed  out  in  connection  with  the  study  of  the 
a  rays,  which  are  only  slightly  deviable,  that  the  radium 
radiations  contain  rays  which  are  readily  deviated  by  the 
magnetic  field.  This  was  shown  by  means  of  an  experi- 
ment already  referred  to  in  connection  with  the  study  of 
a  rays. 

Some  radium  bromide  was  placed  on  the  bottom  of  a 
tube  of  lead,  which  in  turn  was  introduced  between  the  poles 
of  an  electromagnet.  In  front  of  the  tube,  and  at  a  distance 
of  several  centimetres  from  it,  was  an  electroscope.  It  is 
necessary  that  an  air  space  should  intervene  between  the 
tube  and  the  electroscope,  in  order  that  the  a  radiations 
from  the  radium  should  be  cut  of!  and  not  allowed  to  fall 
upon  the  instrument.  A  few  centimetres  of  air  are  quite 
sufficient  to  cut  off  the  easily  absorbed,  non-penetrable 
a  rays,  as  we  have  seen.  The  /3  radiations  from  the  radium 
now  fall  upon  the  electroscope,  together  with  the  y  radia- 
tions; but  since  the  latter  have  only  very  small  power  to 
ionize  a  gas  through  which  they  pass,  they  have  but  little 
power  to  discharge  the  electroscope.  Further,  they  are 
not  deflected  by  a  magnetic  field,  and,  therefore,  their 
action  on  the  electroscope  is  constant  before  and  after  the 
current  is  turned  on  the  electromagnet. 

When  the  electromagnet  is  turned  on  and  a  magnetic 

85 


86        THE  ELECTRICAL  NATURE  OF  MATTER 

field  established,  the  ft  rays  are  readily  deflected  against 
the  walls  of  the  tube,  and  no  longer  fall  on  the  electroscope, 
or  ionize  the  air  between  the  leaves.  The  electroscope  is 
now  discharged  much  more  slowly  than  before  the  magnetic 
field  was  produced.  This  experiment  illustrates  qualita- 
tively the  deviable  nature  of  the  ft  rays. 

A  question  in  this  connection  which  is  of  importance  is 
this:  Are  all  the  ft  rays  equally  deviable?  Are  the  ft 
radiations  homogeneous?  This  is  answered  by  the  follow- 
ing experiments. 

If  in  the  preceding  experiment  the  metal  tube  was  covered 
with  a  metal  plate  having  a  narrow  slit  cut  in  it,  only  a 
narrow  beam  of  rays  could  escape  from  the  tube.  This 
would  produce  only  a  narrow  line  on  a  photographic  plate. 
If  the  magnetic  field  is  now  established,  the  ft  rays  will  be 
deflected  to  one  side.  The  impression  upon  the  plate, 
however,  is  not  that  of  a  displaced  narrow  line,  but  is  a 
broadened  band.  This  shows  that  the  deviable  ft  rays 
are  not  homogeneous,  but  that  some  are  more  deflected 
by  the  magnetic  field  than  others.  They  are  spread  out 
by  the  magnetic  field  into  a  kind  of  spectrum,  showing  that 
some  of  the  ft  particles  have  very  different  velocities  from 
the  others. 

NATURE  OF  THE  CHARGE  CARRIED  BY  THE  BETA  PARTICLES 

The  ft  rays,  as  we  have  seen,  are  deflected  in  the  mag- 
netic field.  The  next  question  is,  are  they  charged,  and  if 
so,  positively  or  negatively?  This  is  answered  by  the 
following  experiment  carried  out  by  M.  and  Mme.  Curie.1 

If  the  ft  rays  are  absorbed  by  any  substance  they  would 
necessarily  give  up  their  charge  to  the  absorbing  medium. 
It  would,  apparently,  be  only  necessary  to  detect  the  nature 

1  Ann.  Chim.  Phys.  [7],  30,  155  (1903). 


THE  BETA  AND  GAMMA  RAYS  87 

of  the  charge  on  the  object  by  which  the  ft  rays  are  absorbed, 
in  order  to  determine  the  nature  of  the  charge  carried  by 
the  ft  rays  themselves. 

While  this  at  first  sight  is  a  very  simple  matter,  a  difficulty 
is  encountered.  The  ft  rays  produce  ions  in  a  gas  through 
which  they  pass.  These  would  conduct  the  charge  away 
from  the  object  upon  which  the  ft  rays  impinge,  and  not 
enough  charge  would  collect  to  be  detected.  In  carrying 
out  such  an  experiment  it  would  obviously  be  necessary  to 
cut  off  the  a  rays  by  means  of  a  thin  sheet  of  metal,  through 
which  the  ft  rays  would  pass,  since  the  a  rays  have  much 
greater  ionizing  power  than  the  ft  rays.  Even  when  this 
is  done  the  ft  rays  render  the  air  a  sufficiently  good  con- 
ductor to  remove  the  electricity  too  rapidly  from  the  object 
which  absorbs  the  ft  rays,  in  order  that  a  sufficient  charge 
should  accumulate  to  be  detected. 

This  difficulty  was  overcome  by  the  Curies  by  imbedding 
the  plate  upon  which  the  ft  rays  were  to  fall,  in  an  insulator 
through  which  the  ft  rays  could  pass.  They  used  thin 
ebonite,  and  also  a  thin  layer  of  paraffine.  The  result  was, 
that  the  Curies  were  able  to  demonstrate  that  the  metal 
upon  which  the  ft  rays  fell,  became  charged  negatively. 
This  proved  that  the  ft  particles  carried  a  negative  charge. 
The  same  result  was  obtained  by  Wien,  who  surrounded 
the  plate  upon  which  the  ft  rays  were  to  fall,  not  with  an 
insulator,  but  with  an  evacuated  vessel. 

The  Curies  proved  that  the  plate  continually  received 
negative  electricity,  as  would  be  expected  by  the  constant 
raining  of  the  negatively  charged  ft  particles  upon  it.  Mme. 
Curie  states  that  only  a  very  weak  current  was  obtained 
under  the  above  conditions,  as  would  be  expected. 

The  Curies  then  undertook  the  sequel  to  the  above  experi- 
ment. If  the  ft  rays  are  charged  negatively,  they  must 


88         THE  ELECTRICAL  NATURE  OF  MATTER 

leave  the  radium  from  which  they  are  shot  off  positively 
charged.  To  test  this  conclusion  the  Curies  placed  the 
radium  salt  in  a  lead  box,  and  surrounded  the  whole  with 
the  insulating  medium.  The  insulating  material  was  then 
surrounded  by  metal  connected  to  earth. 

Under  these  conditions  the  radium  became  positively 
charged,  due  to  negative  charges  being  carried  off  by  the  ft 
particles,  which,  in  this  case,  were  communicated  to  the 
outside  metal  box  and  then  to  earth. 

In  the  above  experiment  the  a  particles  are  completely 
absorbed  by  the  insulated  box,  and  their  effect  thus  re- 
duced to  zero. 

An  interesting  observation  in  this  same  connection  has 
been  described  by  the  Curies.  Radium  would  continue  to 
throw  off  negative  charges  until  it  itself  would  become  so 
highly  charged  positively  that  this  would  prevent  the  further 
sending  off  of  negative  charges.  An  active  preparation  of 
radium  was  sealed  up  for  some  time  in  a  glass  tube.  When 
the  tube  was  scratched  with  a  file,  the  weakened  portion 
was  at  once  perforated  by  a  spark,  and  M.  Curie  at  the  same 
moment  received  an  electric  shock.  The  potential  of  the 
tube  had  thus  been  raised  well  above  the  potential  of  the 
earth,  due  to  the  absorption  of  the  positively  charged  a 
particles,  which  gave  up  their  charge  to  the  inside  of  the 
tube. 

e 

THE  DETERMINATION  OF  —  FOR  THE  BETA  PARTICLE 

m 

We  have  already  studied  the  method  worked  out  by  J.  J. 

^ 
Thomson  for  determining  the  ratio  of  --  for  the  cathode 

particle.  This  method,  it  will  be  remembered,  is  based 
upon  subjecting  the  cathode  rays  to  both  electrostatic  and 


THE  BETA  AND  GAMMA  RAYS  89 

magnetic  deflection.  Exactly  the  same  method  was  used 
with  the  ft  particles  from  radium.  It  is  not  necessary  to 
repeat  the  discussion  of  this  method.  If  necessary,  the 
account  of  the  method  given  in  an  earlier  chapter  should 
be  reread.  The  velocity  of  the  ft  particles,  as  thus  deter- 
mined by  Becquerel,  was  about  i.5Xio10  centimetres  per 

0 

second,  and  the  value  of  —  =  io7.     This  velocity  is  of  the 

m 

same  order  as  that  of  light,  3Xio10  centimetres  per  second, 
and  is  considerably  greater  than  that  found  for  the  cathode 
particle  in  the  low-pressure  tube. 

One  matter  of  very  great  importance  in  this  connection 
must  be  mentioned  again.  It  will  be  remembered  that  all 
of  the  ft  particles  are  not  deflected  equally  by  a  magnetic 
field.  This  was  shown  by  a  broadening  of  the  line  on  the 
photographic  plate,  when  the  magnetic  field  was  produced. 
It  was  pointed  out  that  this  was  due  to  the  fact  that  the  ft 
particles  did  not  all  move  with  the  same  velocity. 

This  is  made  the  basis  of  the  important  experiment  of 
Kaufmann,  to  which  reference  has  already  been  made. 
He  studied  the  electrostatic  and  magnetic  deflections  of 
the  ft  rays  having  different  velocities,  and  determined  the 

value  of  —  for  the  different  rays. 

tn 

He  found  that  this  value  was  not  constant,  but  varied 

(> 
with  the  velocity  o)  the  particle.     The  value  of  --  increased 

as  the  velocity  of  the  particle  diminished.  This  is  seen 
from  the  results,  already  discussed  in  an  earlier  chapter, 
see  page  22. 

The   importance   of  this   observation   has   already   been 

pointed  out.     The  charge  e  carried  by  the  particle  is  con- 

g 

stant,  independent  of  the  velocity.      Since  —  changes  with 

m 


9°         THE  ELECTRICAL  NATURE  OF  MATTER 

the  velocity,  we  must  conclude  that  m,  or  the  mass  oj  the 
particle,  changes  with  the  velocity. 

The  significance  of  this  has  already  been  referred  to  in 
an  earlier  chapter.  It  will  be  remembered  that  the  con- 
clusion to  which  we  were  led,  especially  after  comparing 
the  values  calculated  by  Thomson  with  those  found  experi- 
mentally by  Kaufmann,  is  that  all  mass  is  oj  electrical  origin, 
and  that  matter  is  made  up  of  electrons  or  disembodied 
electrical  charges,  moving  with  high  velocities. 

THE  MASS  OF  THE  BETA  PARTICLE  —  RELATION  TO  THE 
CATHODE  PARTICLE 

The  method  for  determining  the  mass  of  a  particle,  know- 

e 

ing  the  value  of  the  ratio  —  for  it,  has  already  been  dis- 

m 

cussed  at  length.  The  mass  of  the  ft  particles  is  about  yyV^ 
of  the  mass  of  the  hydrogen  ion  in  solutions  of  acids.  //  is, 
therefore,  the  same  as  the  mass  of  the  cathode  particle. 

We  have  now  studied  a  sufficient  number  of  properties 
of  the  ft  rays  to  enable  us  to  make  a  comparison  with  the 
corresponding  properties  of  the  cathode  rays. 

CATHODE  RAYS 

Affect  the  photographic  plate. 

Excite  phosphorescence. 

Ionize  a  gas. 

Are  negatively  charged  particles. 

Have  moderate  power  to  penetrate  matter. 

Have  a  mass  about  yy1^  of  the  mass  of  the  hydrogen  ion. 

Have  a  velocity  about  one-tenth  that  of  light. 

BETA  RAYS  FROM  RADIUM 

Affect  the  photographic  plate. 


THE  BETA  AND  GAMMA  RAYS  91 

Excite  phosphorescence. 

Ionize  a  gas. 

Are  negatively  charged  particles. 

Have  moderate  power  to  penetrate  matter. 

Have  a  mass  about  TyV -5  of  the  mass  of  the  hydrogen  ion. 

Have  a  velocity  that  varies  for  the  different  ft  particles, 
but  the  mean  velocity  is  about  half  that  of  light. 

We  see  from  the  above  that  the  )8  particles  resemble  the 
cathode  particles  very  closely  in  all  of  their  properties, 
except  the  velocity  with  which  they  travel.  That  the  two 
sets  of  particles  should  not  have  the  same  velocities,  is  not  at 
all  surprising,  when  we  consider  the  different  conditions 
under  which  they  are  produced. 

The  f$  particles  are  shot  off  from  radium  with  velocities 
that  are  definite,  and  which  are  conditioned  by  the  nature 
oj  the  substance.  The  cathode  particles  are  shot  off  from 
the  cathode  under  a  high  electrical  stress,  conditioned  in 
part  by  the  difference  between  the  potential  of  the  anode 
and  the  cathode.  Indeed,  we  should  expect  that  the  velocity 
of  the  cathode  particle  would  vary  with  the  field  that  was 
employed,  and  such  is  the  fact.  With  a  strong  field  the 
velocity  of  the  cathode  particle  is  greater  than  with  weak 
fields,  and  with  very  strong  fields  the  velocity  of  the  cathode 
particle  approaches  much  more  nearly  to  the  velocity  of  the 
ft  particle. 

We  can,  then,  regard  the  /B  particles  as  essentially  identical 
with  cathode  particles,  differing  from  them  only  in  the  veloci- 
ties with  which  they  move.  This  would  produce,  as  we 
have  seen,  a  slight  difference  in  the  mass,  but  it  is  not  neces- 
sary to  go  further  into  this  matter  in  the  present  con- 
nection. 

We  have  learned  that  the  cathode  particles  are  nothing 
but  electrons,  or  disembodied,  negative  electrical  charges. 


92        THE  ELECTRICAL  NATURE  OF  MATTER 

Therefore,  the  ft  rays  are  made  up  of  nothing  but  nega- 
tive electrical  charges,  shot  off  from  the  radium  with  enor- 
mous velocities  —  the  velocities  being  comparable  with 
that  of  light. 

We  have  learned  that  all  the  radioactive  substances 
known  give  off  a  particles.  The  three  radioactive  sub- 
stances, uranium,  thorium,  and  radium,  give  off  ft  parti- 
cles. Polonium,  as  we  have  seen,  gives  out  only  a  and  8 
particles. 

SECONDARY  RADIATIONS  PRODUCED  BY  ft  RAYS 

A  very  considerable  amount  of  work  has  been  done 
recently  on  the  absorption  of  ft  rays,  and  on  the  secondary 
radiations  excited  by  them.  McClelland  l  shows  that  the 
secondary  radiation  consists  partly  of  reflected  primary 
rays,  and  partly  of  corpuscles  which  seem  to  have  been 
expelled  from  the  atoms  when  the  primary  rays  entered. 
The  relative  intensities  of  the  secondary  radiations  given 
out  depend  directly  upon  the  atomic  weights  of  the  ele- 
ments upon  which  the  primary  rays  impinge.  Regener 
finds  that  the  ft  rays  produce  scintillations  when  they  fall 
upon  a  screen  of  barium  platinocyanide,  which  is  placed 
between  10  and  50  centimetres  from  the  source  of  the  ft 
rays.  It  will  be  recalled  that  the  a  particles  as  they  pass 
through  matter  lose  energy  gradually  and  finally  cease  to 
ionize  the  gas  through  which  they  are  passing. 

The  absorption  of  the  ft  particles  seems  to  be  quite 
different.  According  to  Makower,2  McClelland  and  Hack- 
ett3  and  others,  the  ft  particles  are  stopped  suddenly,  their 
velocity  just  before  stopping  being  very  high. 

1  Proceed.  Roy.  Soc.,  A,  80,  501. 

2  Trans.  Roy.  Soc.,  cited,  9,  4  (1907). 

3  Phil.  Mag.,  Aug.  (1908). 


THE  BETA  AND  GAMMA  RAYS  93 

THE  GAMMA  RAYS 

A  third  kind  of  rays  is  given  out  by  all  radioactive  sub- 
stances, with  the  exception  of  polonium.  It  was  shown  by 
Villard,  as  we  have  seen,  that  these  rays  are  not  deviated  by 
a  magnetic  field,  and  have  much  greater  power  to  penetrate 
matter  than  either  the  a  or  the  ft  rays.  A  thin  film  of  metal 
is  sufficient  to  stop  the  a  rays.  The  ft  rays  are  all  cut  off 
by  a  piece  of  some  heavy  metal  like  lead  that  is  a  centi- 
metre thick,  while  the  kind  of  rays  with  which  we  are  now 
more  especially  dealing  can,  according  to  Rutherford,  be 
detected  by  a  sensitive  electroscope  after  they  have  passed 
through  a  piece  of  iron  that  is  a  foot  thick. 

These  rays  have  not  as  yet  been  deflected  to  a  detectable 
amount  in  the  magnetic  field. 

While  all  the  radioactive  elements,  with  the  exception 
of  polonium,  give  off  ft  rays,  they  give  them  out  with  very 
different  intensities.  It  would  be  expected  that  the  weakly 
radioactive  elements,  uranium  and  thorium,  would  give 
out  y  rays  to  a  less  extent  than  the  highly  radioactive 
radium,  and  such  is  the  fact.  The  y  rays  given  out  by  the 
weakly  radioactive  elements  have,  however,  been  detected 
by  using  fairly  large  quantities  of  these  subtances. 

The  y  rays,  therefore,  always  accompany  the  ft  rays,  and  this 
is  a  matter  of  importance  in  connection  with  the  theories  that 
have  been  advanced  to  account  for  the  nature  of  the  y  rays. 

Two  hypotheses  as  to  the  nature  of  the  y  rays  have  been 
proposed. 

We  have  seen  that  the  ft  rays  are  made  up  of  electrons, 
or  negative  electrical  charges,  moving  with  different  veloci- 
ties, but  all  having  very  high  velocities ;  the  swiftest  of  these 
travelling  with  a  velocity  which  is  nearly  that  of  light.  It 
is  possible  that  electrons  are  shot  off  from  radium  with  even 


94        THE  ELECTRICAL  NATURE  OF  MATTER 

a  higher  velocity  than  that  of  the  swiftest  ft  rays.  Such 
rays  could  have  at  least  some  of  the  properties  of  the  y  rays. 
Their  great  penetrating  power  might  be  due  to  their  large 
kinetic  energy  resulting  from  their  great  velocity.  The 
fact  that  they  are  not  deflected  in  the  magnetic  field  has  been 
accounted  for  by  the  advocates  of  this  theory,  on  the  ground 
that  the  amount  of  the  deviation  being  an  inverse  function 
of  the  velocity,  the  more  rapidly  moving  particles  might 
be  deflected  to  such  a  small  extent  that  it  would  not  be 
observed.  This  theory  contains  a  number  of  weak  points. 
In  the  first  place,  the  penetrating  power  of  the  y  rays  is  so 
many  times  that  of  the  ft  rays  that  it  seems  difficult  to 
account  for  this  on  the  basis  of  the  slightly  increased  velocity, 
even  if  the  velocity  of  light  is  being  closely  approached. 
Further,  if  this  theory  as  to  the  nature  of  the  y  ray  is  correct, 
we  might  reasonably  expect  to  find  rays  with  penetrating 
power  intermediate  between  that  of  the  ft  ray  and  the  in- 
comparably greater  power  of  the  y  ray.  Indeed,  all  the 
intermediate  stages  could  easily  be  represented.  Such, 
however,  is  not  the  fact.  The  same  criticism  holds  with 
respect  to  the  deviation  in  the  magnetic  field.  If  y  par- 
ticles are  nothing  but  more  rapidly  moving  ft  particles, 
and  if  the  fact  that  the  ft  particles  are  so  readily  deflected 
in  the  magnetic  field,  while  the  y  particles  are  not  deflected 
at  all,  are  to  be  accounted  for  solely  on  the  ground  of  the 
difference  in  velocities,  then  why  do  we  not  find  the  inter- 
mediate stages  represented?  This  question  is  especially 
pertinent  in  consideration  of  the  fact  that  we  do  know  ft 
particles  with  quite  different  velocities.  The  magnetic 
deflection  of  even  the  swiftest  of  these  is  easily  detected. 
If  ft  particles  with  intermediate  velocities  existed,  it  seems 
reasonable  to  think  that  there  would  be  no  serious  difficulty 
in  detecting  their  deflection  in  a  magnetic  field. 


THE  BETA  AND  GAMMA  RAYS  95 

A  theory  as  to  the  nature  of  the  y  rays,  which  accounts 
much  better  for  many  of  the  facts,  is  the  following.  We 
have  seen  in  a  much  earlier  chapter,  that  whenever  cathode 
rays  strike  a  solid  object  X-rays  are  produced.  We  have 
recently  seen  that  the  ft  rays  are  essentially  identical  with 
the  cathode  rays.  We  would  naturally  expect  that  X-rays 
would  be  set  up  where  the  ft  rays  strike  a  solid  object!  The 
ft  rays  from  radium  strike  some  of  the  solid  radium  salt, 
or  some  other  solid,  and  the  y  or  X-ray  is  accordingly 
produced.  The  y  ray,  in  terms  of  this  theory,  is  nothing 
but  an  X-ray.  We  have  seen,  however,  that  it  has  much 
greater  penetrating  power  than  the  X-ray,  and  it  must 
therefore  be  regarded  as  a  very  penetrating  kind  of  X-ray. 

This  theory  accounts  satisfactorily  for  the  entire  absence 
of  deflection  of  the  y  rays  in  a  magnetic  field,  since  ordinary 
X-rays  are  themselves  entirely  undeflected  by  such  a  field. 

This  theory  as  to  the  nature  of  the  y  rays  also  accounts 
for  the  fact  that  y  rays  are  always  absent  unless  ft  rays 
are  present. 

Some  objections  have,  however,  been  offered  to  this 
theory  as  to  the  nature  of  the  y  rays,  so  that  it  must  not 
be  regarded  as  final. 

According  to  Madsen l  the  y  rays  of  radium  and  possibly 
those  of -thorium  consist  of  two  distinct,  homogeneous 
bundles.  When  a  stream  of  y  rays  penetrates  a  metal 
plate,  secondary  y  radiations  appear  on  both  sides  of  the 
plate.  The  amount  of  the  secondary  radiation  from  the 
two  sides  of  the  plate  differs  very  greatly.  A  change  in  the 
hardness  of  the  y  rays  produces  a  marked  difference  in 
the  relative  intensities  of  the  emergent  secondary  radia- 
tion from  various  elements.  These  secondary  radiations, 
however,  do  not  follow  the  order  of  the  atomic  weights. 

^hil.  Mag.  (1907);  Nature  (1908). 


g6        THE  ELECTRICAL  NATURE  OF  MATTER 

SUMMARY  OF  THE  PROPERTIES  OF  THE  ALPHA,  BETA,  AND 
GAMMA  RAYS 

The  a  rays  are  given  off  by  all  radioactive  substances. 
They  are  somewhat  deflected  in  a  magnetic  field.  They 
have  very  small  penetrating  power,  being  easily  absorbed 
even  by  very  thin  layers  of  matter.  They  have  great  power 
to  ionize  a  gas,  rendering  it  a  conductor.  The  a  rays  ionize 
to  about  one  hundred  times  the  extent  of  the  ft  and  y  rays 
together.  They  have  but  little  effect  on  a  photographic 
plate,  but  produce  phosphorescence  in  certain  substances, 
especially  zinc  sulphide.  The  existence  of  phenomena 
such  as  those  manifested  in  the  spinthariscope  are  due 
almost  entirely  to  the  a  particles.  The  a  particle  has  a 
mass  of  the  order  of  magnitude  about  twice  that  of  the 
hydrogen  ion.  This,  however,  is  only  an  approximation. 
The  a  particle  carries  two  positive  charges  of  electricity, 
and  moves  with  a  velocity  about  one-tenth  that  of  light. 

The  ft  rays  are  given  off  from  all  radioactive  substances, 
with  the  exception  of  polonium.  They  are  very  easily  de- 
flected in  a  magnetic  field.  They  are  absorbed  by  matter,  but 
not  near  so  easily  as  the  a  rays.  They  have  comparatively 
small  power  to  ionize  a  gas.  They  do  not  have  great  power  to 
affect  a  photographic  plate,  and  while  they  can  produce  phos- 
phorescence are  less  active  in  this  respect  than  the  a  particles. 

The  (3  particle  has  a  mass  about  yyV^  of  the  mass  of  the 
hydrogen  ion  in  solution,  which  is  the  mass  of  the  electron. 
The  ft  particle  carries  a  unit  charge  of  negative  electricity, 
or,  more  accurately  expressed,  is  a  unit  negative  charge  of 
electricity,  shot  off  with  an  average  velocity  which  is  of 
the  same  order  as  that  of  light.  The  ft  ray  is  practically 
identical  with  the  cathode  ray  in  a  vacuum  tube,  differing 
from  it  chiefly  in  the  velocity  with  which  the  particles  move. 

The  y  rays  exist  where  the  ft  rays  exist.    They  are  not 


THE  BETA  AND  GAMMA  RAYS  97 

deflected  at  all  in  a  magnetic  field.  They  have  very  great 
penetrating  power,  enough  passing  through  a  foot  of  iron  to 
be  detectable  by  the  electroscope.  They  have  much  smaller 
power  than  the  a  particles  to  ionize  a  gas.  They  have  consid- 
erable power  to  affect  a  photographic  plate,  much  greater 
than  the  a  or  even  the  ft  particles.  They  excite  phosphor- 
escence. The  most  probable  theory  as  to  the  nature  of  the 
y  rays  is  that  they  are  a  very  penetrating  form  of  X-ray, 
produced  by  the  ft  rays.  They  are,  therefore,  pulses  in  the 
ether,  set  up  by  the  impact  of  the  ft  rays  on  solid  matter. 

TOTAL  NUMBER  OF  PARTICLES   SHOT  OFF  BY  RADIUM 

Rutherford  determined  the  total  number  of  particles 
shot  off  by  radium.  To  determine  the  total  number  of  a 
particles  he  must  get  rid  of  the  ft  particles.  He  did  this 
by  removing  the  emanation  and  all  of  its  successive  de- 
composition products,  and  obtained  radium  at  what  is 
known  as  its  minimum  activity.  Under  these  conditions 
he  found  that  the  number  of  a  particles  shot  off  per  second 
from  a  gram  of  radium  is  6.2Xio10.  The  number  of  a 
particles  shot  off  by  normal  radium  in  radioactive  equi- 
librium is  approximately  the  same  as  the  number  of  ft 
particles  shot  off  under  the  same  conditions;  since  radium, 
the  emanation,  radium  A,  radium  C,  and  radium  F  all 
emit  a  particles,  while  radium,  and  radium  B,  C  and  D  and 
E  emit  ft  particles.  Radium,  however,  at  its  minimum 
activity  is  freed  from  the  emanation  and  all  succeeding 
decomposition  products,  and  gives  off  the  same  number  of 
a  particles  as  normal  radium  gives  off  ft  particles.  This  also 
was  tested  by  Rutherford.  He  found  that  the  number  of 
ft  particles  shot  off  per  second  from  one  gram  of  radium  was 
7.3  X  io10.  This  is  almost  identical  with  the  number  of  a 
particles  at  minimum  activity. 


CHAPTER  X 
OTHER  PROPERTIES  OF  THE  RADIATIONS 

WE  have  already  studied  a  number  of  the  properties  of 
the  several  kinds  of  radiations,  and  have  compared  the  one 
with  the  other.  We  shall  now  take  up  certain  special 
properties  of  the  several  kinds  of  radiations  sent  out  by 
radium,  as  pointed  out  by  Mme.  Curie.1 


i 


THE  SELF-LUMINOSITY  OF  RADIUM  COMPOUNDS 

While  the  comparatively  pure  radium  salts  give  out  only 
a  little  light,  radium  salts  which  contain  a  large  amount  of 
barium  are  strongly  self-luminous.  This  fact  was  observed 
by  the  Curies.  The  dehydrated,  dry,  halogen  compounds 
of  radium  are  especially  self-luminous.  While  the  self- 
luminosity  cannot  be  perceived  in  ordinary  daylight,  it  can 
be  seen  by  gaslight.  The  self-luminosity  comes  from  the 
entire  mass  of  the  radium  salt,  and  not  simply  from  the 
surface.  In  the  presence  of  moist  air  the  salt  loses  a  large 
amount  of  its  self-luminosity,  but  this  is  again  regained  on 
drying  the  preparation.  The  self-luminosity  persists  for  a 
long  time.  Specimens  preserved  for  years  in  the  dark  still 
continue  to  be  self-luminous.  Mme.  Curie  points  out  that 
the  color  of  the  light  emitted  from  strongly  radioactive 
preparations  changes  with  time,  becoming  more  violet  and 
decreasing  in  intensity.  The  original  intensity  and  color 
are  regained  by  recrystallizing  the  salt  from  water.  The 

1  Ann.  Chim.  Phys.  [7],  30,  145  (1903). 
98 


OTHER  PROPERTIES  OF  THE  RADIATIONS  99 

luminosity  of  the  radium  salt  is  apparently  independent  of 
temperature.  Solutions  of  radium  salts  are  slightly  self- 
luminous.  The  crystals  in  such  a  solution  are  more  strongly 
self-luminous  than  the  solution,  and  can  be  seen  by  the 
light  which  they  emit. 

Mme.  Curie  also  points  out  that  radium  is  the  only  sub- 
stance known  that  is  self-luminous .  It  will  be  remembered 
that  radium  is  the  only  substance  known  that  has  the  power 
to  charge  itself  electrically. 

PHOSPHORESCENCE  PRODUCED  BY  RADIUM  SALTS 

That  salts  of  radium  are  capable  of  exciting  phosphores- 
cence in  certain  substances  has  already  been  mentioned. 
This  was  first  discovered  by  the  Curies.  It  was  subse- 
quently studied  by  others,  and  especially  by  Becquerel. 
Thus,  the  diamond,  ruby,  the  sulphide  of  calcium,  zinc 
sulphide,  barium  platinocyanide,  paper,  glass,  etc.,  have 
been  tested. 

The  action  of  the  radium  is,  however,  not  the  same  as 
that  of  the  X-ray  in  producing  phosphorescence.  Certain 
substances  phosphoresce  when  exposed  to  the  X-ray,  that 
do  not  in  the  presence  of  radium,  and  vice  versa.  In  this 
respect  the  action  of  radium  resembles  more  closely  that  of 
ultra-violet  light. 

Paper,  cotton,  as  well  as  certain  varieties  of  glass  phos- 
phoresce in  the  presence  of  radium.  This  is  especially  true 
of  Thuringian  glass.  Under  the  action  of  radium  the 
glass  that  phosphoresces  becomes  colored  violet  to  brown. 
When  the  glass  has  become  colored  its  power  to  phosphoresce 
is  diminished.  If  the  glass  which  has  become  colored  and 
has  lost  its  power  to  phosphoresce  is  heated,  the  color  is  lost 
and  the  power  is  again  regained.  Barium  platinocyanide 
is  the  best  substance  with  which  to  study  this  action  of 


loo  THE  ELECTRICAL  NATURE  OF  MATTER 

radium  salts.  It  shows  phosphorescence  when  placed  two 
metres  from  active  radium. 

Zinc  sulphide,  as  we  have  seen,  is  also  rendered  phos- 
phorescent by  the  radium  rays.  It  is  especially  sensitive 
to  the  action  of  the  a  rays,  where  it  shows  the  characteristic 
scintillations  in  the  spinthariscope.  It  has  already  been 
mentioned  that  the  diamond  becomes  phosphorescent  in 
the  presence  of  radium,  and  can  thus  be  distinguished  from 
the  imitation. 

While  all  three  kinds  of  rays  produce  phosphorescence, 
the  a  rays,  on  the  whole,  are  the  most  active.  This  can  be 
seen  by  interposing  between  the  radium  and  the  screen 
a  thin  piece  of  metal  foil  or  of  paper  which  will  cut  off  the 
a  particles.  The  ft  and  y  rays  can  also  produce  phos- 
phorescence, especially  in  screens  of  barium  platinocyanide. 
Their  power  is,  however,  much  feebler  than  that  possessed 
by  the  a  particles. 

RADIUM    INCREASES    THE    CONDUCTIVITY    OF    DIELECTRICS 

The  property  of  radium  to  ionize  a  gas  and  render  it  a 
conductor  has  already  been  repeatedly  mentioned.  A  good 
qualitative  method  of  demonstrating  this  power  is  the 
following:  Take  an  induction  coil  and  place  the  discharging 
points  just  so  far  apart  that  a  spark  will  cease  to  pass.  Then 
place  a  glass  tube  containing  a  few  milligrams  of  an  active 
radium  salt  between  the  two  points.  The  discharge  will  take 
place  at  once.  This  is  due  to  the  ionization  of  the  air  be- 
tween the  terminals  by  the  radiation  from  the  radium.  In 
the  above  case  most  of  the  ionization  is  produced  by  the  y 
rays,  since  most  of  the  a  and  ft  rays  are  cut  off  by  the  glass. 
If  the  radium  salt  were  placed  in  an  open  vessel  so  as  to  se- 
cure the  ionizing  effect  of  the  strongly  ionizing  a  rays,  the 
conductivity  of  the  gas  would  be  still  more  increased. 


OTHER  PROPERTIES  OF  THE  RADIATIONS  IOI 

The  conductivity  of  a  number  of  liquid  non-conductors  is 
very  considerably  increased  by  exposing  them  to  the  radium 
radiations.  Thus,  a  number  of  our  best  liquid  insulators 
acquire  a  measurable  conductivity  under  the  influence  of 
the  radiations  from  radium.  This  applies  to  carbon  disul- 
phide,  petroleum  ether,  liquid  air,  vaseline  oil,  etc. 

It  would  seem  that  this  ionization  in  liquids  was  pro- 
duced mainly  by  the  y  radiations,  since  similar  results 
were  obtained  by  M.  Curie  when  the  liquids  were  exposed  to 
X-rays,  with  which,  it  will  be  remembered,  the  y  rays  are 
closely  allied.  Similar  results  have  been  obtained  with 
certain  solid  dielectrics.  Thus,  p'araffine  exposed  to  the 
radiations  from  radium  acquires  some  conductivity.  The 
ionization  produced  in  the  paraffine,  as  well  as  in  the  liquid 
non-conductors,  is  probably  due  mainly  to  the  more  pene- 
trating rays  from  radium. 

CHEMICAL  EFFECTS  PRODUCED  BY  RADIOACTIVE  SUBSTANCES 

The  crystalline  halogen  salts  of  the  alkalies  —  the  chlo- 
rides, bromides,  etc.,  are  colored  by  radium  radiations  as  by 
cathode  rays.  The  Curies  observed  that  glass  and  porce- 
lain became  colored  when  exposed  to  radium.  A  violet  or 
brown  color  appears  in  the  glass,  which  persists  after  the 
removal  tff  the  radium.  Glass  which  has  been  exposed  for 
a  considerable  time  to  the  action  of  radium  becomes  dark- 
ened. This  is  apparently  true  of  all  glasses. 

Mme.  Curie  subjected  a  number  of  glasses  of  known,  but 
widely  different  composition,  to  the  action  of  the  radium 
radiations,  and  concluded  that  the  coloration  was  due  to 
the  presence  of  the  alkali  metal  in  the  glass.  Salts  of  the 
alkali  metals  themselves  showed  more  vivid  coloration,  and 
a  greater  variety  of  colors  than  the  different  glasses  that 
were  studied  by  Mme.  Curie. 


102        THE  ELECTRICAL  NATURE  OF  MATTER 

The  most  probable  theory  as  to  the  cause  of  the  coloration 
in  glass  is  that  the  radiations  from  radium  liberate  the  alkali 
metals,  which  then  form  a  solid  solution  in  the  glass. 

Radium  transforms  oxygen  into  ozone,  which  can  be 
detected  by  its  odor.  This  is  due  to  the  a  and  f$  rays,  since, 
when  these  are  cut  off,  no  ozone  is  produced.  To  understand 
what  this  transformation  really  means,  we  must  ask  the 
question,  what  is  the  real  difference  between  oxygen  and 
ozone?  The  older  text-books  on  chemistry  state  that  the 
difference  in  the  properties  of  oxygen  and  ozone  is  to  be 
referred  to  the  fact  that  oxygen  contains  two  atoms  in  the 
molecule,  and  ozone  three.  It  is  obvious  that  this  explains 
nothing,  except  the  difference  between  the  mass  of  the  atom 
of  oxygen  and  the  mass  of  the  atom  of  ozone.  The  chemi- 
cal and  physical  properties,  in  general,  of  substances  cannot 
be  explained  on  any  material  bases.  To  gain  any  rational 
conception  of  them  we  must  take  into  account  the  energy 
relations  and  conditions  that  exist  in  the  substance  in  ques- 
tion. 

It  is  a  simple  matter  to  prove  that  the  real  difference 
between  the  properties  of  oxygen  and  ozone  is  due  to  the 
different  amounts  of  intrinsic  energy  possessed  by  their 
molecules.  If  we  burn  carbon  in  oxygen  or  in  ozone,  the 
same  end  product,  carbon  dioxide,  is  obtained.  If  oxygen 
and  ozone  contain  different  amounts  of  intrinsic  energy, 
there  will  be  different  amounts  of  heat  liberated  when  the 
same  amounts  of  carbon  are  burned  in  the  two  gases;  since 
the  amount  of  heat  liberated  in  any  case  is  the  thermal  ex- 
pression of  the  difference  between  the  intrinsic  energy  of 
the  system  before  a  reaction  has  taken  place,  and  after  the 
reaction  is  completed. 

If  we  burn  a  given  weight  of  carbon  in  ozone,  more  heat 
is  liberated  than  when  we  burn  the  same  weight  of  carbon 


OTHER  PROPERTIES  OF  THE  RADIATIONS  103 

in  oxygen.  Since  the  same  amounts  of  carbon  dioxide  are 
formed  in  the  two  cases,  we  must  conclude  that  ozone  con- 
tains more  intrinsic  energy  than  oxygen,  and  any  differences 
in  the  properties  of  these  two  allotropic  modifications  of  the 
same  element  are  to  be  referred  to  the  different  amounts 
of  intrinsic  energy  possessed  by  their  molecules.  Radium, 
then,  adds  energy  to  oxygen,  transforming  it  into  ozone, 
and  this  is  accomplished  mainly  by  the  a  and  ft  rays.  This 
is  in  keeping  with  our  knowledge  of  the  radiations  given 
off  from  radium,  since  most  of  the  energy  is  contained  in 
the  a  particles.  According  to  Becquerel,  radium  radia- 
tions can  also  transform  white  phosphorus  into  red. 

Radium  compounds  undergo  changes  themselves  under 
their  own  radiations.  When  the  method  of  separating 
radium  from  pitchblende  was  under  discussion,  it  was 
pointed  out  that  crystals  of  radium  chloride  with  which 
barium  chloride  was  mixed,  while  colorless  when  first 
formed,  became  quickly  colored.  The  color  is  lost  by 
recrystallizing  the  salt.  The  coloration  produced  by  the 
radium  salts  extends  more  deeply  into  the  substance  than 
that  caused  by  the  cathode  rays. 

It  has  already  been  mentioned  that  the  radiations  from 
radium  affect  a  photographic  plate.  This  is,  of  course, 
due  to  alchemical  action  on  the  silver  salt  of  the  photo- 
graphic plate.  Polonium  acts  on  a  photographic  plate 
only  when  the  plate  is  brought  very  near  to  the  substance. 
This  is  due  to  the  fact  that  polonium  gives  out  only  a  rays, 
which  have  weak  photographic  action;  and  further,  are 
largely  absorbed  by  a  layer  of  air,  even  a  few  centimetres 
in  thickness. 

Radium,  however,  acts  at  much  greater  distance  on  a 
photographic  plate.  It  produces  a  marked  impression  at 
a  distance  of  several  feet,  even  when  the  radium  is  inclosed 


104        THE  ELECTRICAL  NATURE  OF  MATTER 

in  a  glass  tube,  which  cuts  off  all  of  the  a  rays,  and  some  of 
the  fi.  We  have  seen  that  it  is  the  y  rays  that  are  especially 
active  photographically.  It  has  been  found  that  the  best 
radiographs  are  produced  by  the  y  rays  alone. 

PHYSIOLOGICAL   ACTION    OF    THE    RADIATIONS    FROM   RADIUM 

Fairly  active  radium  is  capable  of  producing  burns  or 
wounds  when  brought  near  the  skin,  that  are  both  painful 
and  slow  to  heal.  The  skin  is  first  inflamed  and  reddened, 
and  may  actually  become  blistered  if  exposed  for  a  sufficient 
length  of  time  close  to  an  active  preparation  of  radium. 

The  action  of  the  radiations  from  radium  upon  certain 
diseases  of  the  skin,  such  as  lupus,  has  been  tested,  and 
apparently  has  yielded  good  results  in  the  hands  of  the 
dermatologist.  It  has  also  been  claimed  to  have  produced 
wholesome  effects  upon  cancerous  tissue,  especially  in  the 
early  stages.  Whether  it  is  really  capable  of  curing  this 
disease  remains  to  be  seen.  It  is  certainly  true  that  the 
radiations  from  radium  are  more  penetrating  than  ultra- 
violet light  or  X-rays,  which  have  been  shown  to  have  cer- 
tain curative  properties.  They  can,  therefore,  penetrate 
more  deeply  into  the  tissue,  and  might  give  better  results. 

An  interesting  physiological  experiment  has  been  studied 
by  Himstedt  and  Nagel.1  If  a  preparation  of  radium  is 
brought  near  the  closed  eye  in  a  dark  room,  a  sensation  of 
light  is  produced.  This  is  due  to  the  phosphorescence 
produced  within  the  eye  itself  by  the  radium,  the  lens  and 
retina  being  strongly  phosphorescent  under  the  action  of 
the  13  and  y  rays.  This  sensation  is  experienced  even  by 
the  blind,  if  the  retina  has  not  been  destroyed. 

Aschkinass  and  Caspari  have  shown  that  the  radiations 
from  radium  also  diminish  the  activity  of  certain  bacteria. 

1  Ann.  d.  Phys.,  4,  537  (1901). 


OTHER  PROPERTIES  OF  THE  RADIATIONS  105 

A  large  number  of  facts  in  connection  with  the  action  of 
radium  upon  living  matter  have  been  brought  to  light. 
It  would  obviously  lead  too  far  to  discuss  these  at  length  in 
the  present  connection. 

The  physiological  action  of  radium  is  due  mainly  to  the 
a  and  ft  rays.  These  are  cut  off  by  placing  the  radium  salt 
in  a  metal  box  —  especially  in  one  of  lead.  This  precau- 
tion should  always  be  taken  when  active  preparations  of 
radium  are  being  used.1 

1  For  further  details  in  reference  to  the  matters  discussed  in  this  chapter, 
see  the  article  by  Mme.  Curie  in  Ann.  Chim.  Phys.  [7],  30,  186-203 


CHAPTER  XI 
PRODUCTION  OF  HEAT  BY  RADIUM  SALTS 

AN  observation  of  the  greatest  importance  was  made  in 
1903  by  M.  Curie  and  Laborde.1  Salts  of  radium  have  a 
temperature  that  is  continually  above  that  oj  the  surrounding 
medium.  This  means  that  heat  is  being  produced  in  the 
radium  compound.  That  the  radium  salt  is  warmer  than 
the  surrounding  air  can  be  shown  qualitatively  by  means  of 
fairly  sensitive  mercury  thermometers.  It  can  be  readily 
demonstrated  in  the  following  manner,  according  to  Mme. 
Curie.  A  double-walled  glass  bulb  was  made,  and  the 
space  between  the  two  walls  exhausted.  The  object  of 
removing  the  air  was  to  render  the  space  between  the  walls  a 
very  poor  conductor  of  heat.  Into  such  a  vacuum- jacketed 
vessel  the  bromide  of  radium,  placed  in  a  glass  tube,  was 
introduced,  together  with  a  relatively  sensitive  thermometer. 
Into  a  second  such  vessel  a  similar  thermometer  was  intro- 
duced. The  thermometer  placed  near  0.7  of  a  gram  of  the 
radium  salt  registered  two  or  three  degrees  higher  than 
the  thermometer  in  the  vessel  that  contained  no  radium. 
Thus,  quite  appreciable  differences  in  temperature  were 
produced  with  a  few  decigrams  of  the  radium  compound. 
With  larger  quantities  of  the  salt  still  greater  differences 
in  temperature  would  result. 

MEASUREMENT     OF     THE     HEAT     LIBERATED     BY     SALTS 
OF  RADIUM 

Several  methods  have  been  employed  to  measure  the 
quantity  of  heat  liberated  in  a  given  time,  by  a  given  quantity 

1  Compt.  rend.,  136,  673  (1903). 
106 


PRODUCTION  OF  HEAT  BY  RADIUM  SALTS  107 

of  radium.  A  rough  method  carried  out  by  M.  Curie  and 
Dewar  is  more  novel  and  interesting  than  important.  It 
is  well  known  that  Dewar,  provided  with  the  splendid  low- 
temperature  plant  of  the  Royal  Institution,  has  been  able 
to  obtain  in  large  quantities  all  of  the  lowest  condensing 
gases,  with  the  exception  of  helium,  in  the  liquid  form. 

He  has  obtained  liquid  hydrogen  in  considerable  quantity, 
and  worked  out  a  number  of  its  interesting  properties.  He 
has  determined  its  boiling-point,  and  found  this  to  be  only 
about  twenty  on  the  absolute  scale,  which  is  —253  degrees 
centigrade.  If  heat  is  added  to  liquid  hydrogen  it  will 
boil.  On  account  of  the  very  low  temperature  at  which 
liquid  hydrogen  boils,  it  will  take  up  heat  from  any  surround- 
ing liquid  except  more  of  the  liquid  hydrogen  itself,  and 
would  thus  continue  to  boil  without  cessation,  or  at  least  to 
give  off  appreciable  quantities  of  hydrogen  gas. 

A  test-tube,  whose  lower  half  was  surrounded  by  a  double- 
walled,  vacuum  jacket,  was  filled  about  one-third  full  with 
liquid  hydrogen.  This  was  then  immersed  in  a  larger 
vessel,  also  surrounded  by  a  double-walled  vacuum  jacket, 
and  the  space  between  the  two  filled  with  liquid  hydrogen. 
The  hydrogen  in  the  inner  tube  soon  ceased  to  give  off  any 
appreciable  amount  of  gas,  since  it  could  not  obtain  the 
heat  necessary  to  convert  itself  into  vapor  —  the  conduc- 
tion of  heat  being  prevented  by  the  hydrogen  in  the  outer 
vessel,  which  always  continued  to  give  off  gas.  If  any  heat 
was  supplied  to  the  liquid  hydrogen  in  the  inner  vessel,  a 
part  of  the  liquid  would  be  converted  into  vapor  which 
would  escape. 

The  experiment  consisted  in  arranging  the  system  as 
above  described,  and  waiting  until  the  gas  ceased  to  escape 
from  the  inner  vessel.  A  weighed  quantity  of  the  radium 
salt,  sealed  up  in  a  glass  tube,  was  then  introduced  into  the 


108        THE  ELECTRICAL  NATURE  OF  MATTER 

liquid  hydrogen  in  the  inner  tube.  The  tube  and  salt  being 
at  ordinary  temperatures  when  introduced  into  the  liquid 
hydrogen,  would  give  up  heat  to  the  liquid  until  they  were 
cooled  down  to  the  temperature  of  the  liquid  hydrogen 
itself.  This  would,  of  course,  volatilize  a  part  of  the  liquid, 
and  gaseous  hydrogen  would  escape.  After  the  small  glass 
tube  containing  the  radium  salt  and  its  contents  had  been 
cooled  to  the  temperature  of  the  liquid  hydrogen,  gas  would 
cease  to  escape  from  this  tube,  unless  the  radium  gave  off 
heat.  In  fact,  gas  continued  to  escape  from  the  tube,  as 
long  as  any  liquid  hydrogen  remained  in  the  vessel.  This 
was  due  to  the  heat  being  given  off  continuously  by  the 
radium. 

It  is  obvious  that  the  amount  of  hydrogen  gas  set  free  in  a 
given  time  can  be  used  to  measure  the  rate  at  which  heat 
is  being  liberated  by  the  radium.  It  is  only  necessary  to 
collect  the  hydrogen  and  measure  it  by  any  of  the  methods 
for  measuring  a  gas,  and  to  determine  the  heat  of  vaporiza- 
tion of  hydrogen,  i.e.,  the  amount  of  heat  required  to  pro- 
duce, say,  100  cubic  centimetres  of  hydrogen  gas,  from  the 
liquid.  Weighing  the  amount  of  pure  radium  salt  that  was 
introduced  into  the  liquid  hydrogen,  we  have  all  the  data 
necessary  for  calculating  the  rate  at  which  radium  liberates 
heat,  or  the  amount  of  heat  produced  by  a  given  quantity 
of  radium  in  a  given  time.  While  this  method  is  far  less 
accurate  than  the  one  to  be  described  subsequently,  it  is 
useful  as  a  confirmatory  check;  and  interesting  when  we 
think  that  the  liquid  which  is  vaporized  by  the  heat  spon- 
taneously produced  by  radium  is  one  that  was  unknown 
until  the  last  few  years,  and  one  which  defied  the  skill  of 
so  many  able  experimenters  to  produce,  including  the 
immortal  Faraday. 

This  method  of  measuring  the  amount  of  heat  liberated 


PRODUCTION  OF  HEAT  BY  RADIUM  SALTS  109 

by  radium  has  one  feature  which  is  of  special  importance. 
The  radium  is  giving  off  heat,  under  these  conditions,  at 
the  temperature  of  liquid  hydrogen,  which  is  only  about 
twenty  degrees  centigrade  above  the  absolute  zero.  By 
comparing  the  results  of  this  method  with  those  of  methods 
that  can  be  employed  at  ordinary  temperatures,  we  can 
see  what  effect  temperature  has  on  the  rate  of  heat  produc- 
tion by  radium. 

If  the  production  of  heat  in  salts  of  radium  is  due  to  any 
chemical  action,  we  should  expect  that  the  rate  at  which 
heat  is  evolved  by  radium  would  be  greatly  lessened  at  the 
very  low  temperature,  since  nearly  all  chemical  reactions 
take  place  more  slowly  the  lower  the  temperature.  Indeed, 
most  chemical  reactions  fail  to  take  place  at  all  at  the  tem- 
perature of  liquid  hydrogen. 

It  has  been  found  that  radium  liberates  just  as  much  heat 
at  the  temperature  oj  liquid  hydrogen,  as  at  ordinary  tem- 
peratures. This  alone  makes  it  highly  improbable  that  the 
heat  liberated  by  radium  in  its  salts  is  due  to  any  chemical 
action  taking  place  within  the  compound.  We  shall  see 
later  that  the  amount  of  heat  liberated  by  salts  of  radium 
is  of  an  order  of  magnitude  so  much  greater  than  that  known 
in  the  case  of  any  chemical  reaction,  that  this  source  of  the 
heat  energy  is  almost  certainly  excluded.  Further,  the  fact 
that  salts  of  radium  continue  to  produce  heat  for  apparently 
an  almost  indefinite  time,  excludes  the  possibility  that  it  is 
produced  as  the  result  of  chemical  action. 

METHOD  OF  THE  BUNSEN  ICE  CALORIMETER 

The  amount  of  heat  liberated  by  salts  of  radium  is  meas- 
ured most  accurately  by  means  of  the  Bunsen  ice  calorim- 
eter. The  principle  of  this  instrument  is  so  well  known 
that  only  a  few  words  of  explanation  are  necessary.  The 


HO  THE  ELECTRICAL  NATURE  OF  MATTER 

essential  feature  of  this  method  is  the  use  of  a  block  of  ice, 
which  is  melted  by  the  heat  that  it  is  desired  to  measure. 
Knowing  the  amount  of  ice  converted  into  water  and  the 
heat  of  fusion  of  ice,  we  have  all  the  data  necessary  for 
determining  the  amount  of  heat  set  free  in  the  ice  calorim- 
eter. 

In  some  of  the  earlier  work  with  the  Bunsen  ice  calorim- 
eter, the  amount  of  water  produced  was  obtained  by 
collecting  it  and  then  weighing  it.  A  more  accurate  method 
of  determining  the  amount  of  ice  that  has  been  melted  is 
based  upon  the  fact  that  the  ice  and  the  resulting  water 
occupy  different  volumes.  When  water  freezes  the  volume 
increases,  and,  conversely,  when  ice  melts  the  volume  occu- 
pied by  the  resulting  water  is  less  than  that  occupied  by  the 
ice.  This  principle  is  utilized  to  determine  the  amount  of 
the  ice  melted. 

RESULTS  OF  HEAT  MEASUREMENTS 

The  results  are  certainly  surprising  on  account  of  their 
enormous  magnitude.  A  gram  of  radium  gives  out  every 
hour  about  eighty  calories  o)  heat.  Since  the  heat  of  fusion 
of  ice  is  eighty  calories,  or  eighty  calories  of  heat  are  re- 
quired to  melt  one  gram  of  ice,  it  follows  that  radium  gives 
out  enough  heat  to  melt  its  own  weight  oj  ice  every  hour. 

The  most  remarkable  feature  of  all,  is  the  fact  that  radium 
continues  to  give  out  heat  at  this  rate  for  apparently  an 
indefinite  time.  We  shall  see  later  that  this  would  go  on  as 
long  as  the  radium  itself  continues  to  exist. 

This  is  a  most  surprising  result.  Indeed,  it  is  one  of  the 
most  startling  facts  that  has  ever  been  discovered  in  any 
branch  of  physical  science.  Think  of  the  enormous  amount 
of  energy  that  this  substance  is  capable  of  liberating! 


PRODUCTION  OF  HEAT  BY  RADIUM  SALTS  ill 

SOURCE  OF  THE  HEAT 

The  question  naturally  arose  whence  came  this  energy? 
Some  rushed  to  the  conclusion  that  it  must  be  created  by 
the  radium,  and  that  the  law  of  the  conservation  of  energy 
was  overthrown.  Those  who  were  less  radical  concluded 
that  radium  must  have  the  power  to  transform  some  un- 
known kind  of  energy  into  heat,  which  was  essentially  the 
same  as  to  admit  that  they  did  not  know,  and  had  no  tangi- 
ble conception  of  the  origin  of  this  energy. 

The  more  conservative  began  to  look  around  for  a  rational 
explanation  of  this  astonishing  and  most  important  fact,  in 
the  light  of  what  was  known,  or  what  could  be  discovered. 

We  shall  see  a  little  later  that  their  efforts  were  rewarded, 
and  that  we  have  a  rational  explanation  as  to  the  origin  of 
the  enormous  amount  of  energy  given  out  by  radium. 

We  have  seen,  then,  that  very  large  quantities  of  energy 
are  liberated  by  the  element  radium,  and  that  this  con- 
tinues unabated  for  practically  an  unlimited  time. 

The  heat  is  given  off  slowly,  compared  with  the  heat  that 
is  given  out  in  certain  combustions.  This  is  the  reason 
that  the  radium  salt  does  not  heat  itself  to  a  higher  tem- 
perature above  the  surrounding  medium.  Another  ex- 
planation-of  why  larger  differences  in  temperature  do  not 
exist,  is  that  such  small  quantities  of  radium  salts  have  thus 
far  been  obtained,  that  the  heat  is  lost  by  conduction  through 
the  relatively  large  surface  exposed  to  surrounding  objects. 
If  large  amounts  of  radium  could  be  obtained,  it  is  quite 
certain  from  the  rate  at  which  heat  would  be  produced, 
that  the  interior  of  a  pile  of  radium  chloride  or  bromide 
would  become  quite  hot;  and  by  suitably  surrounding  the 
salt  with  a  medium  that  was  a  poor  conductor  of  heat,  .it 
is  quite  possible  that  the  interior  of  a  pile  of  radium  salt 


112        THE  ELECTRICAL  NATURE  OF  MATTER 

might  become  red-hot  and  actually  give  off  light,  due  to 
the  heat  spontaneously  produced  by  itself. 

EFFECT  ON  SOLAR  HEAT 

The  fact  that  radium  gives  out  heat  energy  has  been 
utilized  to  explain  certain  natural  phenomena,  for  which  a 
satisfactory  explanation  has  long  been  wanting.  Take  the 
heat  of  the  sun,  how  is  it  produced  ?  A  number  of  theories 
have  been  advanced.  The  possibility  of  the  heat  of  the 
sun  being  the  result  of  combustion  or  any  chemical  action 
has  long  since  been  abandoned.  A  similar  fate  has  be- 
fallen the  theory  that  solar  heat  is  produced  by  meteoric 
bodies  raining  down  from  space  on  to  the  sun.  Both  of 
these  views  have  been  found  to  be  insufficient  in  the  light 
of  well-known  facts. 

The  theory  that  is  held  to-day  is  that  the  origin  of  solar 
heat  is  to  be  found  in  the  contraction  that  is  going  on  in  the 
sun  itself.  This  contraction  would,  of  course,  produce  a 
constant  shrinking,  and  a  dropping  in  of  the  exterior,  which 
would  give  rise  to  heat;  and  in  the  case  of  a  body  of  the 
dimensions  of  the  sun,  would  give  rise  to  enormous  amounts 
of  heat. 

This  theory  is  to  be  sharply  distinguished  from  the  older 
one,  that  the  sun  is  simply  a  cooling  body,  giving  out  solar 
heat  as  it  cools.  According  to  the  present  theory  enormous 
amounts  of  heat  are  being  continually  produced  in  the  sun, 
while  according  to  the  cooling  theory  the  sun  is  simply 
giving  out  heat  like  any  other  hot  body. 

This  theory  of  the  origin  of  solar  heat  has  been  found  to 
account  for  the  facts.  A  contraction  which  would  be  too 
small  to  be  observed  during  the  time  that  careful  solar 
measurements  have  been  made,  would  account  for  all  the 
heat  given  out  by  the  sun  during  this  period. 


PRODUCTION  OF  HEAT  BY  RADIUM  SALTS  113 

While  this  theory  is  capable  of  accounting  for  solar  heat, 
there  has,  however,  been  a  reservation  in  the  minds  of 
men  of  science,  which  has  made  them  hesitate  to  accept 
the  theory  as  the  final  explanation  of  the  origin  of  all  solar 
heat. 

The  discovery  of  the  large  amount  of  heat  liberated  by 
radium  has  been  utilized  by  Rutherford1  to  account  for  at 
least  a  part  of  the  solar  heat.  If  the  sun  consists  of  a  very 
small  fraction  of  one  per  cent,  of  radium,  this  would  account 
for  the  heat  that  is  given  out  by  it. 

The  fundamental  question  in  connection  with  this  theory 
as  to  the  origin  of  all  or  part  of  the  solar  heat  is  this:  Does 
the  sun  contain  radium?  Is  there  any  evidence,  direct  or 
indirect,  that  radium  exists  in  the  sun? 

It  must  be  said  that  no  direct  evidence  has  as  yet  been 
produced  to  show  the  presence  of  radium  in  the  sun.  The 
supposed  discovery  of  the  spectrum  lines  of  radium  in  the 
sun  leaves  much  to  be  desired.  The  supposed  coincidences 
of  the  solar  lines  with  the  known  lines  of  radium  are  only 
rough  approximations.  Indeed,  so  rough  that  they  are  far 
from  being  convincing. 

DOES  RADIUM  EXIST  IN  THE  SUN? 

Indirect  evidence  of  the  presence  of  radium  in  the  sun, 
however,  exists.  It  has  been  shown  by  spectrum  analysis 
that  helium  exists  in  the  sun.  Indeed,  this  element  was 
first  discovered  in  the  sun,  as  its  name  implies.  It  was  only 
recently  discovered  by  Ramsay  as  occurring  at  all  on  the 
earth.  We  shall  see  that  helium  and  radium  are  most 
closely  associated.  Wherever  we  find  the  one,  we  may 
reasonably  expect  the  other.  Helium,  having  been  shown 
to  exist  in  considerable  quantities  in  the  sun,  the  conclusion 

i  Phil.  Mag.,  5,  591  (1903). 


114  THE  ELECTRICAL  NATURE  OF  MATTER 

is  highly  probable  that  the  sun  also  contains  radium.  The 
force  of  this  argument  will  appear,  and  be  the  better  appre- 
ciated, when  the  exact  relation  of  helium  and  radium  is 
taken  up  in  a  later  chapter.  The  hypothesis  of  the  radium 
origin  of  even  a  part  of  the  solar  heat  is  only  an  hypothesis, 
which  it  will  remain  for  the  future  either  to  raise  to  the 
rank  of  a  theory,  or  to  disprove. 

TERRESTRIAL    HEAT    PRODUCED    BY    RADIUM  —  BEARING    ON 
THE  CALCULATED  AGE  OF  THE  EARTH 

We  have  seen  that  radium  exists  widely  scattered  over 
the  surface  of  the  earth.  While  only  small  quantities  have 
been  found  in  any  one  place,  and  while,  in  the  opinion  of 
the  writer,  for  reasons  already  expressed,  this  is  likely  to 
continue  to  be  the  case,  yet  the  total  amount  of  radium  in 
the  earth  may  be  very  considerable.  Indeed,  there  are 
reasons  for  supposing  that  beneath  the  surface  of  the  earth 
there  may  be  more  radium  than  on  the  surface.  The 
waters  from  certain  springs,  which  probably  come  from 
considerable  depths,  contain  radium.  All  of  this  radium 
is  continually  giving  out  heat. 

Rutherford  points  out  that  the  heat  liberated  by  radium 
in  the  earth  may  have  an  appreciable  effect  on  its  age  as 
usually  calculated.  In  such  calculations,  starting  with  the 
earth  as  a  molten  mass,  the  main  factors  that  are  taken 
into  account  in  addition  to  the  original  temperature  are; 
the  specific  heat  of  the  earth  to  determine  how  much  heat 
it  contains,  and  the  conductivity  of  the  crust  of  the  earth 
for  heat,  to  determine  the  rate  at  which  the  earth  is  losing 
heat.  Given  these  data,  the  problem  is  to  determine  how 
long  it  would  require  the  earth  to  cool  from  the  condition 
of  a  molten  mass  to  its  present  state. 

In  this  calculation  it  is  not  assumed  that  there  is  any 


PRODUCTION  OF  HEAT  BY  RADIUM  SALTS  115 

large  source  of  heat  production  going  on  within  the  earth 
itself.  The  hydration  of  the  rocks,  or  the  combination  of 
the  rocks  with  water  as  they  cool,  would  liberate  some  heat, 
and  this  is  taken  into  account.  If,  however,  it  should  be 
shown  that  there  is  an  appreciable  quantity  of  radium  in 
the  earth,  this  would  give  off  heat  continuously,  and  in 
geological  time  the  amount  of  heat  from  this  source  might 
be  very  considerable,  relative  to  the  total  heat  in  the  earth 
itself.  This  factor  might  vitiate  the  calculation  of  the  age 
of  the  earth  on  the  basis  of  the  data  that  have  been  used, 
and  produce  a  very  considerable  error  in  the  result.  The 
magnitude  of  the  error  would,  of  course,  depend  entirely 
upon. the  amount  of  radium  in  the  earth. 

THEORIES  AS  TO  THE   SOURCE  OF  THE  HEAT  PRODUCED   BY 

RADIUM 

Several  theories  have  been  advanced  to  account  for  the 
production  of  the  heat  that  is  continuously  being  liberated 
by  radium.  One  is  strictly  analogous  to  the  contraction 
theory  of  solar  heat.  The  radium  atom  is  contracting  or 
shrinking  up,  and  heat  is  therefore  produced.  This  theory, 
which  never  met  with  much  favor,  is  now  untenable,  for 
reasons  that  will  appear  as  the  subject  develops. 

The  theory  as  to  the  origin  of  heat  in  the  salts  of  radium, 
which  accounts  satisfactorily  for  the  facts,  and  which  is 
now  generally  accepted,  is  the  following.  We  have  seen 
that  the  a  particles  shot  out  by  radium  are  incapable  of 
penetrating  any  appreciable  thickness  of  matter.  They  are 
all  absorbed  by  thin  screens.  We  have  also  seen  that  these 
particles  have  a  mass  at  least  twice  that  of  the  hydrogen 
atom,  and  possibly  greater,  and  are  shot  out  at  very  high 
velocities.  These  particles  would,  therefore,  have  large 
amounts  of  kinetic  energy,  and  when  they  are  stopped  this 


Il6        THE  ELECTRICAL  NATURE  OF  MATTER 

would  be  transformed  into  heat  and  would  yield  a  large 
amount  of  it. 

Take  a  pile  of  radium  salt,  the  a  particles  shot  off  from 
the  surface,  not  coming  in  contact  with  any  of  the  salt  above 
it,  would  escape  at  least  a  few  centimetres  into  the  air.  But 
the  a  particles  shot  off  from  all  of  the  radium  at  any  appre- 
ciable distance  beneath  the  surface  of  the  salt  would  not 
escape,  but  would  strike  the  solid  salt  above  it  and  be 
stopped.  The  energy  of  motion  of  the  a  particle  would 
thus  become  converted  into  heat.  Since  the  mass  of  the  a 
particle  is  considerable,  and  the  velocity  about  one- tenth 
that  of  light,  the  kinetic  energy  would  be  great,  and  the 
amount  of  heat  produced  considerable. 

This  theory,  which  was  proposed  by  Lodge,1  to  account 
for  the  heat  liberated  by  radium,  as  produced  by  the  stop- 
ping of  the  a  particles  in  their  flight,  leaves  still  one  question 
unanswered.  How  do  the  a  particles  acquire  this  great 
velocity  with  which  they  are  shot  off  from  the  radium  ? 

We  can  scarcely  conceive  of  particles  at  rest  in  a  molecule 
being  shot  off  with  such  velocities.  The  particles  in  the 
molecule  or  atom  of  radium  —  the  electrons  —  must  be 
moving  with  very  high  velocities,  and  when  a  particle  in  its 
motion,  gets  beyond  the  control  of  the  attractions  of  the 
remaining  particles  of  the  system,  it  flies  off.  This  is  true 
of  the  positively  charged  a  particles,  and  also  of  the  nega- 
tively charged  f$  particles.  The  kinetic  energy  of  these 
particles  is  then  something  inherent  in  the  atom  of  ra- 
dium. This  we  call  intrinsic  energy.  It  is  obvious  that 
this  is  the  real  source  of  the  heat  liberated  by  radium. 
The  astonishing  feature  is  the  amount  of  the  intrinsic 
energy  contained  in  the  atoms  of  radium. 

1  Nat.,  67,  511  (1903). 


PRODUCTION  OF  HEAT  BY  RADIUM   SALTS  1 17 

CALCULATION  OF  THE  AMOUNT  OF  HEAT  LIBERATED  BY  RA- 
DIUM, ON  THE  ABOVE  THEORY  THAT  THE  HEAT  IS  PRO- 
DUCED BY  THE  a  PARTICLES 

Rutherford  l  also  points  out  that  from  the  number  of  a 
particles  expelled  from  radium  we  can  calculate  the  heating 
effect,  since  this  is  due  to  the  bombardment  of  the  a  par- 
ticles. He  calculated  the  kinetic  energy  of  the  a  particle 
to  be  5.9X10-°  ergs.  Radium  at  its  minimum  activity 
gives  off,  as  we  have  seen,  6.2  X  io10  a  particles  from  a  gram 
per  second.  In  radioactive  equilibrium  it  gives  off  4X6.2  X 
io10  =  2.5X10"  a  particles  per  gram-second.  This  would 
correspond  for  a  gram  of  radium  to  126  gram-calories  per 
hour.  The  value  found  was  80,  which  agrees  well  with  the 
above  calculation. 

THREE   REMARKABLE   PROPERTIES   OF  RADIUM 

We  have  thus  far  met  with  at  least  three  properties 
possessed  by  radium,  which  are  in  the  highest  degree 
remarkable. 

(1)  We  have  seen  that  radium  has  the  power  to  charge 
itself  electrically. 

(2)  It  also  has  the  power  to  illuminate  itself,  or  is,  as  we 
say,  self-luminous. 

(3)  We  have  just  seen  that  radium  produces  heat  energy 
spontaneously,  or  can  warm  itself. 

These  three  properties  alone  would  suffice  to  place  radium 
in  a  class  by  itself. 

1  Phil.  Mag.,  io,  206  (1905). 


CHAPTER  XII 
EMANATION  FROM  RADIOACTIVE  SUBSTANCES 

WE  have  already  seen  that  many  radioactive  substances 
give  off  a  particles,  which  are  positively  charged,  material 
bodies.  Many  radioactive  substances,  polonium  being  an 
exception,  give  off  ft  particles,  which  are  negative  charges 
of  electricity  or  electrons,  having  the  same  mass  as  the 
negative  charges  in  the  cathode  ray,  i.e.,  about  ifV^  °f  the 
mass  of  the  hydrogen  ion  in  solution.  All  radioactive  sub- 
stances which  give  off  f$  particles  also  give  off  7  rays. 
This  includes  many  radioactive  substances,  a  marked  excep- 
tion being  polonium.  The  7  rays  are  probably  identical 
with  the  X-rays,  except  that  they  are  far  more  penetrating. 

We  have  also  seen  that  radium  gives  out  continuously 
large  quantities  of  heat.  Since  this  production  of  heat 
energy  is  due  mainly  to  the  a  particles,  it  seems  fair  to 
assume  that  all  radioactive  substances  that  give  off  a  par- 
ticles, and  this,  as  was  just  stated,  includes  most  of  them, 
also  give  off  heat  energy.  In  the  case  of  the  weakly  radio- 
active elements,  such  as  uranium  and  thorium,  the  number 
of  a  particles  given  off  is  relatively  small,  and,  therefore,  the 
amount  of  heat  energy  given  off  by  them  is  relatively  slight. 
It  may,  indeed,  be  so  slight  as  to  escape  detection. 

In  addition  to  these  three  kinds  of  radiations,  and  the 
heat,  certain  radioactive  elements,  such  as  thorium,  radium, 
and  actinium,  give  off  what  Rutherford  calls  an  emanation. 
This  substance,  as  we  shall  see,  resembles  in  many  respects 

118 


EMANATION  FROM  RADIOACTIVE  SUBSTANCES  119 

a  gas.  It  can  diffuse  through  porous  bodies,  can  be  con- 
densed at  low  temperature,  etc.  It  has  in  general  the 
properties  of  the  radioactive  substances  from  which  it  was 
obtained. 

DISCOVERY  OF  THE  THORIUM  EMANATION  BY  RUTHERFORD 

The  amount  of  the  emanation  given  off  even  by  radium 
is  small,  and  for  some  time  escaped  detection.  We  owe  its 
discovery  in  fact  to  the  study  of  the  radioactivity  of  thorium. 
It  had  been  observed  by  Mme.  Curie  and  others  that  the 
radioactivity  of  thorium  was  not  constant  when  the  thorium 
compound  was  placed  in  a  vessel  exposed  to  air  currents. 
If  the  compound  of  thorium,  on  the  other  hand,  was  placed 
in  a  closed  vessel,  constant  results  could  be  obtained.  It 
was  found  that  the  lack  of  constant  results  in  open  vessels 
was  due  to  air  currents.  If  a  current  of  air  was  drawn 
through  the  closed  vessel  containing  the  thorium,  incon- 
stant results  were  again  obtained.  Rutherford  l  took  up 
the  study  of  the  cause  of  this  irregularity,  and  the  result 
was  the  discovery  of  the  emanation. 

METHOD  OF  OBTAINING  THE  EMANATION 

The  emanation  can  be  obtained  from  the  salts  of  radium 
by  simply  heating  them,  or  by  dissolving  them  in  water, 
when  it  is  given  off,  the  admixed  carbon  dioxide  being 
absorbed  by  potassium  hydroxide.  It  can  be  collected  in  a 
vessel  like  any  other  gas,  and  its  properties  studied.  Before 
taking  up  its  general  physical  and  chemical  properties,  one 
property  especially  will  be  discussed  in  some  detail,  since 
it  practically  demonstrates  the  gaseous  nature  of  this 
substance.  The  emanation  can  be  condensed  at  low  tem- 
peratures, like  an  ordinary  gas,  into  a  liquid.2 

1  Phil.  Mag.,  49,  i  (1900). 

2  Rutherford  and  Soddy:  Phil.  Mag.,  5,  561  (1903). 


120        THE  ELECTRICAL  NATURE  OF  MATTER 

If  hydrogen  is  allowed  to  bubble  through  a  solution  of  a 
radium  salt,  and  is  then  passed  through  a  U-tube  surrounded 
by  liquid  air,  the  emanation  condenses  in  the  tube.  Similar 
results  are  obtained  if  the  products  expelled  by  heating  a 
radium  salt  are  passed  through  a  U-tube  dipped  in  liquid 
air. 

If  only  a  small  amount  of  the  radium  salt  is  available, 
the  condensation  of  the  emanation  is  shown  by  the  fact  that 
the  escaping  hydrogen  is  either  not  radioactive  at  all,  or 
only  slightly  so;  while  the  emanation  is  extremely  radio- 
active. If  a  larger  amount  of  the  emanation  is  obtainable, 
its  presence  in  the  cold  glass  tube  can  be  seen;  not  by  pro- 
ducing under  ordinary  conditions  a  visible  amount  of  liquid, 
but  by  a  fluorescence  in  the  air  in  the  cold  tube,  and  also  by 
rendering  the  walls  of  the  tube  brilliantly  phosphorescent. 

By  a  modification  of  the  above- described  experiment,  it  is 
possible  to  determine  the  temperature  at  which  the  emana- 
tion condenses  or  boils,  A  mixture  of  the  emanation  and  a 
neutral  gas  is  passed  through  a  tube  cooled  down  below 
the  temperature  at  which  the  emanation  condenses.  When 
the  emanation  was  all  condensed,  the  escaping  gas,  hydro- 
gen, oxygen,  nitrogen,  or  air  showed  no  radioactivity  when 
tested  by  the  electrical  method.  After  all  the  emanation 
had  been  condensed,  a  current  of  neutral  gas,  say  hydrogen, 
was  passed  through  the  tube  containing  the  emanation. 
The  temperature  in  the  condensing  tube  gradually  rose,  due 
to  the  presence  of  the  warmer  gas,  and  when  the  boiling- 
point  of  the  emanation  was  reached  and  a  little  of  it  was 
volatilized,  its  radioactivity  manifested  itself  in  deflecting  the 
electrometer  with  which  the  vessel  into  which  the  emanation 
passed  was  connected.  When  this  took  place,  the  tempera- 
ture in  the  condensing  vessel  was  read  by  means  of  a  copper 
resistance  thermometer  that  had  been  previously  calibrated. 


EMANATION  FROM  RADIOACTIVE  SUBSTANCES  1 21 

The  average  result  from  a  number  of  experiments  showed 
that  the  emanation  condenses  at  —152  degrees  centigrade. 
This  point  was  fairly  sharply  determined  by  the  fact  that 
the  ionization  or  conductivity  of  the  gas,  into  which  the 
escaping  emanation  passed,  reached  a  maximum  shortly 
after  the  emanation  began  to  volatilize,  and  when  the 
temperature  had  been  raised  only  a  very  slight  amount. 

The  emanation  thus  condenses  to  a  liquid  just  like  a  gas, 
and  like  a  gas  has  a  perfectly  definite  boiling-point. 

AMOUNT  OF  THE  EMANATION 

The  amount  of  the  emanation  obtainable  even  from  an 
appreciable  quantity  of  radium  is  very  small  indeed.  If 
the  emanation  that  can  be  obtained  from  a  tenth  of  a  gram 
of  radium  chloride  or  bromide  is  condensed  in  a  glass  tube 
as  previously  described,  no  liquid  or  even  mist  will  be  seen 
in  any  part  of  the  tube.  All  that  will  be  seen  is  a  phos- 
phorescence on  the  walls  of  the  tube,  and  this  may  extend 
through  the  neutral  gas  within  the  tube. 

Sir  William  Ramsay  and  Soddy  have  measured  approxi- 
mately the  volume  of  the  emanation  obtainable  from  a 
given  quantity  of  the  radium  salt.  The  emanation  was 
collected  in  a  capillary  tube  which  had  been  graduated,  and 
measured. 

From  one  gram  of  radium  they  obtained  one  cubic  milli- 
metre of  the  gas.  Rutherford  found  the  value  0.59  cubic 
millimetre  per  gram  of  radium.  This  volume  decreased 
rapidly  with  time,  and  we  shall  learn  that  this  is  a  very 
important  fact. 

Rutherford  also  points  out  that  knowing  the  number  of 
a  particles  shot  off  from  radium,  we  can  calculate  the 
volume  of  the  em-anation  produced  by  it.  Every  atom  of 
radium  in  breaking  up  gives  off  at  least  one  a  particle  and 
produces  one  atom  of  the  emanation  which  is  a  gas.  A 


122  THE   ELECTRICAL  NATURE   OF   MATTER 

cubic  centimetre  of  a  gas  is  known  to  contain  about  3.6X 
io19  molecules.  From  these  data  the  volume  of  the  emana- 
tion that  can  be  obtained  from  a  gram  of  radium  is  cal- 
culated to  be  0.83  cubic  millimetre.  The  volume  of  the 
emanation  from  a  gram  of  radium,  as  found  experimentally 
by  Ramsay  and  Soddy  as  already  stated,  was  one  cubic 
millimetre.  The  two  results,  when  we  consider  the  condi- 
tions, are  strikingly  concordant. 

NATURE   OF   THE   EMANATION 

In  studying  the  properties  of  the  emanation  we  encounter 
the  great  difficulty,  which  at  present  is  insurmountable, 
that  it  cannot  be  obtained  in  appreciable  quantity.  This 
is  especially  true  of  the  emanation  from  thorium,  as  might 
be  expected  from  the  small  radioactivity  of  this  element. 
We  have  just  seen  that  the  emanation  from  radium  dis- 
appears, or  "  decays,"  as  it  is  said,  quite  rapidly.  This  is 
especially  true  of  the  emanation  from  thorium,  which  is  not 
only  infinitesimal  in  quantity,  but  disappears  or  decays  in 
a  few  minutes.  The  emanation  from  radium,  however, 
does  not  entirely  decay  for  a  number  of  days. 

The  emanation  itself  is  unaffected  by  an  electrostatic 
field,  and  is,  therefore,  not  charged.  It  can,  however, 
produce  phosphorescence  in  certain  substances. 

After  having  shown  that  the  emanation  has  many  of  the 
properties  of  gases,  and  is  certainly  material  in  nature, 
attempts  were  made  by  Rutherford  to  identify  it  with  some 
of  the  known  substances.  Its  chemistry  was  studied  as  far 
as  possible  with  the  small  quantity  available.  It  was  sub- 
jected to  very  high  temperatures,  but  was  unaffected  by  this 
treatment.  Then  it  was  passed  through  a  platinum  tube 
heated  as  highly  as  the  nature  of  the  tube  would  permit.  It 
was  also  passed  over  heated  platinum  black,  and  escaped 


EMANATION  FROM  RADIOACTIVE   SUBSTANCES          123 

in  both  cases  without  change.  In  the  above  experiments 
the  emanation  was  mixed  with  air.  It  was  then  mixed 
with  hydrogen  and  passed  over  red-hot,  magnesium  pow- 
der, and  also  over  red-hot  palladium,  but  it  was  still  un- 
affected. 

Ramsay  sparked  a  mixture  of  the  emanation  with  oxygen, 
for  a  long  time,  in  the  presence  of  an  alkali,  and  also  heated 
it  in  the  presence  of  magnesia  lime,  but  the  emanation  was 
unchanged.  The  emanation  thus  differs  from  all  known 
forms  of  matter,  except  argon  and  the  other  members  of 
this  group  of  elements,  which  are  characterized  by  their 
chemical  inertness. 

While  we  do  not  know,  even  at  present,  very  much  about 
the  chemistry  of  the  emanation,  it  seems  safe  to  conclude 
that  if  it  is  an  element  it  belongs  to  that  inactive  group  of 
chemical  elements  of  which  argon  was  the  first  member  to 
be  discovered.  Even  if  it  should  be  shown  not  to  be  ele- 
mentary, it  nevertheless  resembles  in  its  chemical  proper- 
ties the  elements  of  this  group. 

Some  light  has  been  thrown  on  the  physical  properties 
of  the  emanation,  notwithstanding  the  fact  that  it  has  been 
obtained  only  in  such  small  quantities. 

DIFFUSION     OF     THE     EMANATION  —  APPROXIMATE    DETER- 
MINATION OF  ITS  MOLECULAR  WEIGHT 

It  is  well  known  that  gases  diffuse  with  very  different 
velocities.  If  we  allow  gases  of  different  densities  to  dif- 
fuse into  any  gas,  say  the  atmospheric  air,  we  shall  find  not 
only  that  they  will  diffuse  with  very  different  velocities,  but 
a  regularity  will  manifest  itself.  The  lighter  gases  will 
diffuse  more  rapidly  than  the  heavier  ones. 

If  we  work  quantitatively,  we  shall  find  a  very  simple 


124       THE  ELECTRICAL  NATURE  OF  MATTER 

relation  between  the  densities  or  the  molecular  weights  of 
gases,  and  the  rates  at  which  they  will  diffuse. 

Gases  diffuse  with  velocities  that  are  inversely  proportional 
to  the  square  roots  of  their  densities. 

This  generalization,  known  from  its  discoverer  as  the 
law  of  Graham,  is  comprehensive,  holding  for  all  well- 
known  gases. 

Upon  the  basis  of  this  generalization,  Rutherford  and 
Miss  Brooks  1  have  attempted  to  determine  approximately 
the  molecular  weight  of  the  emanation  from  radioactive 
substances,  notwithstanding  the  fact  that  the  largest 
amount  of  the  emanation  thus  far  obtained  is  scarcely 
weighable  even 'with  the  most  refined  chemical  balance. 

They  allowed  the  emanation  to  diffuse  from  one  end  of 
a  tube  into  the  other,  and  measured  the  change  in  the 
conductivity  of  the  air  in  the  tube. 

From  the  data  thus  obtainable  the  diffusion  coefficient 
of  radium  could  easily  be  calculated. 

The  experiments  which,  on  the  whole,  were  the  most 
satisfactory  and  probably  the  most  accurate,  gave  a  diffu- 
sion coefficient  which  was  close  to  0.07. 

If  we  compare  this  coefficient  with  the  diffusion  coeffi- 
cients of  vapors  whose  molecular  weights  are  known,  we 
find  that  it  comes  close  to  the  coefficient  for  ether,  which 
has  the  value  of  0.077,  and  the  molecular  weight  of  ether  is 
74.  The  molecular  weight  of  the  emanation  from  radium 
must,  therefore,  be  close  to  74.  All  things  considered, 
Rutherford  seems  to  think  that  the  molecular  weight  of  the 
radium  emanation  is  not  far  from  100.  The  emanation 
from  thorium  was  shown  to  have  practically  the  same  molec- 
ular weight  as  the  emanation  from  radium. 

Since  the  above  determinations  of  the  molecular  weight 

1  Chem.  News,  85,  196  (1902). 


EMANATION  FROM  RADIOACTIVE   SUBSTANCES          125 

of  the  radium  emanation  were  made  by  Rutherford  and 
Miss  Brooks,  new  determinations  have  been  carried  out 
by  Makower,1  working  with  J.  J.  Thomson. 

Radium  bromide  was  dissolved  in  water  and  the  emana- 
tion removed  by  passing  air  through  the  solution.  The 
mixture  of  air  and  the  emanation  was  collected  over  mer- 
cury in  one  arm  of  a  glass  vessel  resembling  a  Hempel 
burette,  which  was  closed  at  the  top  by  a  porous  plug.  This 
vessel,  known  as  the  diffusion  vessel,  was  connected  with  a 
cylindrical  brass  vessel.  Into  the  centre  of  this  brass  cylin- 
der a  brass  rod  was  introduced,  so  as  to  be  insulated  from 
the  walls  of  the  vessel.  By  means  of  a  storage  battery  of 
two  hundred  cells,  a  difference  in  potential  of  about  four 
hundred  volts  was  established  between  the  brass  rod  and 
the  walls  of  the  box.  A  known  volume  of  the  mixture  of 
air  and  the  emanation  was  introduced  from  the  diffusion 
vessel  into  the  brass  cylinder,  and  the  conductivity  of  the 
gases  in  the  cylinder  determined.  As  soon  as  the  conduc- 
tivity had  been  determined,  the  emanation  was  quickly 
pumped  out  of  the  cylinder,  so  as  to  minimize  the  amount 
of  the  "  induced  radioactivity  "  on  the  walls  of  the  vessel, 
which  quickly  decayed. 

The  mixture  of  air  and  the  emanation  now  gradually 
diffused  out  of  the  diffusion  vessel,  through  the  porous  plug. 
From  time  to  time  fresh  quantities  of  the  mixture  were  driven 
over  from  the  diffusion  vessel  into  the  brass  cylinder,  and  its 
conductivity  determined.  As  more  and  more  of  the  emana- 
tion diffused  out  through  the  porous  plug  in  the  top  of  one 
arm  of  the  diffusion  vessel,  the  conductivity  of  the  mixture 
remaining  in  the  vessel  became  less  and  less,  as  was  shown 
by  testing  the  conductivity  at  short  intervals,  by  the  method 
already  described.  In  this  way  it  was  not  difficult  to  deter- 

1  Phil.  Mag.,  9,  56  (1905). 


126  THE   ELECTRICAL   NATURE   OF   MATTER 

mine  the  rate  at  which  the  emanation  diffused  out  through 
the  porous  plug. 

To  determine  the  molecular  weight  of  the  emanation,  it 
was  necessary  to  compare  its  rate  of  diffusion  with  that 
of  gases  whose  molecular  weights  were  known,  diffusing 
through  the  same  porous  plug.  The  gases  employed  were 
oxygen,  hydrogen,  carbon  dioxide,  and  sulphur  dioxide. 
The  gas  was  introduced  into  the  diffusion  vessel  and 
allowed  to  diffuse  out  into  the  atmosphere.  Knowing  the 
molecular  weights  of  the  gases,  the  rates  at  which  they 
diffuse  through  the  given  porous  plug,  and  the  rate  at 
which  the  emanation  diffuses  through  the  same  plug,  we 
can  calculate  the  molecular  weight  of  the  emanation  from 
Graham's  law. 

The  results  showed  a  molecular  weight  for  the  radium 
emanation  ranging  from  85.5  to  99.  On  the  assumption 
that  the  radium  emanation  is  a  monatomic  gas,  Makower 
points  out  that  this  result  would  give  it  a  place  in  the 
Periodic  System  in  the  fluorine  group  between  molybdenum 
and  ruthenium. 

The  molecular  weight  of  the  emanation  from  thorium 
was  found  to  be  slightly  smaller  than  that  from  radium. 

These  results  show  that  the  molecular  weight  of  the 
emanation  is  very  nearly  one  hundred,  as  Rutherford  had 
supposed. 

Ramsay  and  Gray  1  determined  the  molecular  weight  of 
the  radium  emanation  which  they  called  niton  as  220. 
Later 2  they  found  223. 

1  Compt.  rend.;  151,  126  (1910). 

2  Pro.  Roy.  Soc.  84,  536  (1911). 


CHAPTER  XIII 
HELIUM  PRODUCED  FROM  THE  EMANATION 

WE  have  seen  that  the  emanation  is  material,  and  has 
many  of  the  properties  of  an  ordinary  gas.  We  have  also 
seen  that  when  the  emanation  is  present  in  the  radium,  the 
latter  gives  out  a,  ft  and  y  radiations.  The  question  arises 
whether  the  emanation  gives  out  all  three  types  of  rays,  or 
only  certain  special  types,  or  does  it  give  out  any  radiation 
at  all? 

This  was  tested  by  Rutherford  and  Soddy  l  in  the  follow- 
ing way.  The  thorium,  containing  the  emanation,  was 
placed  in  a  metal  box,  having  a  hole  in  the  top  that  was 
covered  with  a  plate  of  mica.  The  radiation  from  the 
emanation  that  passed  through  the  mica  was  tested  by  its 
power  to  ionize  the  gas  above  it.  When  a  thin  metal  disk 
was  interposed  in  the  path  of  the  radiation,  most  of  the 
radiation  was  cut  off.  This  showed  that  at  least  most  of 
the  radiation^  consisted  of  a  rays.  No  evidence  was  ob- 
tained that  any  ft  rays  were  present. 

In  the  case  of  the  emanation  from  radium,  the  test  as  to 
its  nature  was  made  as  follows:  The  emanation  was  intro- 
duced into  a  copper  tube,  whose  walls  were  thick  enough 
to  cut  off  all  the  a  rays.  No  ft  or  y  rays  were  given  out  by 
the  emanation  itself. 

The  emanation  gives  out,  then,  only  one  type  of  radia- 
tion, and  that  is  the  a  type.  No  ft  or  y  rays  come  from  the 
emanation  either  from  thorium  or  radium.  It  will  be  re- 

i  Phil.  Mag.,  5,  445  (1903)- 
127 


128       THE  ELECTRICAL  NATURE  OF  MATTER 

membered  that  the  a  rays  are  composed  of  positively  charged 
particles,  having  a  mass  about  four  times  that  of  the  hydro- 
gen atom,  and  moving  with  a  velocity  which  is  about  one- 
tenth  that  of  light.  It  will  also  be  recalled  that  it  is  the  a 
particles  that  have  most  of  the  energy  given  off  by  radioac- 
tive substances,  since  they  have  appreciable  mass  and  very 
high  velocity.  The  a  rays  are  the  chief  agents  that  ionize 
a  gas  subjected  to  radioactive  substances,  and  are  the  most 
important  radiations  given  off  by  such  substances. 

Having  found  that  the  emanation  gives  off  a  particles, 
the  next  question  is,  do  all  the  a  particles  shot  off  from 
radium  come  from  the  emanation  contained  in  it,  or  has 
deemanated  radium  any  power  to  produce  a  rays?  This 
can  easily  be  answered.  When  all  of  the  emanation  is 
^emoved  from  the  radium  salt  by  heating,  the  remaining 
deemanated  radium  also  has  some  power  to  give  out  a 
particles. 

Rutherford  studied  the  effect  of  low  temperature  on  the 
rate  at  which  the  emanation  was  produced.  He  found  that 
the  emanating  power  of  thoria  was  diminished  to  about 
one-tenth  at  the  temperature  of  solid  carbon  dioxide. 

M.  Curie  found  that  the  emanating  power  of  radium 
compounds  was  much  increased  by  dissolving  them  in 
water.  The  meaning  of  some  of  these  empirical  facts  will 
appear  when  we  come  to  study  the  nature  of  the  changes 
that  are  taking  place  in  radioactive  substances. 

RECOVERY  OF  EMANATING  POWER 

When  thorium  or  radium  compounds  are  subjected  to  a 
high  temperature  they  become  deemanated,  or  lose  most 
of  their  emanating  power.  They,  however,  regain  this 
power  on  standing,  more  and  more  of  the  emanation  being 
produced. 


HELIUM  PRODUCED  FROM  THE  EMANATION  129 

DECAY  OF  THE  EMANATION 

If  we  study  the  emanation,  we  find  that  the  activity  of 
the  emanation  rapidly  diminishes.  The  activity  of  the 
emanation  obtained  from  thorium  decreases  to  one-half  its 
initial  value  in  about  one  minute,  and  almost  entirely  van- 
ishes in  a  very  few  minutes. 

The  activity  of  the  radium  emanation  is,  however,  more 
persistent.  The  most  careful  work  on  this  problem  is  un- 
doubtedly that  of  Rutherford  and  Soddy.  A  mixture  of 
the  emanation  with  air  was  preserved  over  mercury,  and 
samples  removed  and  examined  from  time  to  time.  They 
found  that  the  activity  of  the  emanation  from  radium  fell 
to  half  the  initial  value  in  3.85  days. 

The  rate  of  decay  of  the  emanation  seems  to  be  independent 
o]  the  conditions  to  which  the  emanation  is  subjected.  Even 
high  temperatures  have  no  effect  on  the  rate,  and  when  the 
emanation  is  condensed  to  a  liquid  at  low  temperatures, 
the  decay  goes  on  at  the  same  rate. 

HEAT  EVOLVED  BY  THE  EMANATION 

We  have  discussed  at  some  length  in  an  earlier  chapter 
the  remarkable  heat-producing  power  of  radium.  We 
have  seen  that  the  amount  of  heat  liberated  by  radium  is 
one  of  the  most  surprising  facts  in  physical  science. 

We  have  now  studied  in  some  detail  the  unique  substance 
which  is  being  constantly  produced  and  given  off  by  radio- 
active substances,  known  as  the  emanation.  It  is  extremely 
radioactive  considering  its  quantity.  Indeed,  much  of  the 
radioactivity  of  radium  and  thorium  can  be  referred  to  the 
emanation  produced  by  and  contained  in  them. 

We  would  naturally  ask,  does  the  emanation  have  any- 
thing to  do  with  the  enormous  production  of  heat  that  is 
taking  place  in  radium  salts,  and  if  so,  what? 


130       THE  ELECTRICAL  NATURE  OF  MATTER 

The  answer  to  this  question  we  owe  to  Rutherford  and 
Barnes.1  They  worked  with  only  thirty  milligrams  of  the 
bromide  of  radium  and  determined  the  total  heat  emission 
of  this  substance. 

They  then  distilled  off  the  emanation  and  condensed  it  in 
a  tube  surrounded  by  liquid  air.  This  tube  was  sealed  up 
while  immersed  in  the  refrigerating  agent.  The  heat  that 
was  liberated  by  the  emanation  in  the  tube  was  then  meas- 
ured from  time  to  time,  and  also  the  heat  that  was  liberated 
by  the  radium  bromide  from  which  the  emanation  had  been 
distilled.  The  sum  of  the  heat  liberated  by  the  emanation, 
plus  that  liberated  by  the  bromide  from  which  the  emana- 
tion had  been  obtained,  was  always  equal  to  the  total 
amount  of  heat  set  free  from  the  original  bromide. 

When  the  emanation  was  giving  out  a  maximum  amount 
of  heat,  the  surprising  fact  was  established  that  from  seventy 
to  seventy-five  per  cent,  of  the  total  heat  given  out  by  radium 
salts  comes  from  the  emanation  contained  in  them,  and  Ruther- 
ford has  recently  shown  that  about  thirty  per  cent,  of  the 
total  heating  effect  of  radium  comes  from  radium  C,  one  of 
the  decomposition  products  of  the  emanation. 

This  fact  is  even  more  wonderful  than  the  discovery 
that  small  amounts  of  radium  salts  can  give  off  such  large 
amounts  of  heat.  We  have  now  traced  the  source  of  most 
of  this  heat  to  the  almost  infinitesimal  quantity  of  emana- 
tion contained  in  such  small  amounts  of  the  salts  of  radium 
that  are  at  present  at  our  disposal. 

HELIUM  PRODUCED  FROM  THE  EMANATION 

We  have  already  encountered  a  number  of  remarkable 
and  surprising  facts  in  connection  with  the  radioactive 
elements  and  the  emanation  produced  by  them.  Perhaps 
the  most  remarkable  still  remains  to  be  considered.  We 
have  seen  that  the  activity  of  the  emanation  gradually 

1  Phil.  Mag.,  7,  202  (1904). 


HELIUM  PRODUCED  FROM  THE  EMANATION  131 

decays  and  finally  becomes  zero.  This  necessitates  the 
conclusion  that  some  fundamental  change  is  going  on  in 
the  emanation  itself. 

A  number  of  questions  arise  in  this  connection.  Espe- 
cially prominent  is  this  one:  If  the  emanation  is  under- 
going decomposition,  into  what  does  it  decompose?  What 
is  left  in  a  tube  containing  the  emanation  after  the  emana- 
tion has  ceased  to  be  radioactive? 

If  we  go  back  to  pitchblende  —  the  source  of  most  of  our 
radium  —  we  find  such  a  large  number  of  things,  that  it 
would  appear  to  be  difficult  to  say  that  any  one  of  them 
was  a  product  of  the  decomposition  of  the  emanation  from 
the  radium  contained  in  this  mineral.  We,  however,  find 
most  of  these  substances  occurring  in  other  associations 
somewhere  in  nature  where  no  radium  is  present,  and  they, 
therefore,  could  not  be  the  final  product  of  the  decomposi- 
tion of  the  radium  emanation. 

If  we  examine  the  radioactive  minerals  closely  we  shall 
see,  however,  that  they  contain  one  substance,  of  which 
the  above  remark  is,  at  best,  only  partially  true.  This  is 
the  element  helium. 

This  element,  as  has  already  been  pointed  out,  was  first 
discovered  spEctroscopically  in  the  sun  by  Lockyer.  It 
was  first  discovered  among  the  terrestrial  elements  by 
Ramsay.  This  discovery  has  an  interesting  history.  Ram- 
say was  working  with  Lord  Rayleigh  on  argon,  and  had 
studied  its  properties,  and  especially  its  chemical  inertness. 
In  this  connection  it  occurred  to  him  to  examine  the  inert 
gas  previously  obtained  from  the  mineral  cleveite,  to  see 
whether  it  was  not  argon.  He  examined  it  spectroscop- 
ically  and  found  a  prominent  yellow  line  near  the  sodium 
line,  which  he  could  not  identify  as  coincident  with  that 
of  any  known  terrestrial  element.  However,  on  comparing 


132  THE  ELECTRICAL  NATURE  OF  MATTER 

it  with  the  line  discovered  by  Lockyer  in  the  sun,  Ramsay 
found  that  the  two  were  identical. 

Helium  was  thus  shown  to  exist  among  the  terrestrial 
elements. 

It  should  further  be  pointed  out  that  helium,  as  far  as  it 
occurs  at  all  in  minerals,  is  only  to  be  found  in  the  radio- 
active minerals.  Helium  is  also  found  in  the  waters  of 
certain  springs,  but  probably  comes  from  radioactive  min- 
erals which  are  at  some  depth  below  the  surface  of  the 
earth. 

Taking  these  facts  into  account,  and  also  the  chemical 
properties  of  the  emanation  from  thorium  and  radium, 
Rutherford  and  Soddy  l  suggested  that  the  emanation  on 
decomposing  might  yield  some  inert  element  of  the  type 
of  those  in  the  argon  family. 

On  account  of  his  ability  and  experience  in  working  with 
small  quantities  of  gases,  Sir  William  Ramsay  2  undertook 
the  study  of  the  nature  of  the  emanation,  with  the  assist- 
ance of  Mr.  Soddy. 

They  dissolved  from  20  to  30  milligrams  of  radium  bro- 
mide in  water,  and  collected  the  emanation  in  a  sparking 
tube.  The  sparking  tube  was  connected  with  a  U-tube 
which  was  surrounded  by  liquid  air.  This  condensed  any 
carbon  dioxide  that  was  present  in  the  emanation  as  an 
impurity,  and  also  the  emanation.  If  any  helium  was  pro- 
duced from  the  emanation,  this  would  not  be  condensed 
by  the  liquid  air,  since  helium  liquifies  at  a  lower  tempera- 
ture than  air. 

When  the  spectrum  of  this  tube  was  taken,  a  bright  yellow 
line  made  its  appearance,  which  was  not  far  removed  from 
the  sodium  line;  but  even  with  a  small  spectroscope  could 

1  Phil.  Mag.,  4,  581  (1902). 

2  Nat.,  68,  246  and  354  (1903). 


HELIUM  PRODUCED  FROM  THE  EMANATION  133 

be  seen  not  to  be  identical  with  it.  A  careful  measurement 
of  this  line  showed  it  to  be  identical  with  the  D3  line  oj  helium. 
This  preliminary  experiment  with  its  remarkable  result, 
led  to  further  very  careful  work  on  the  problem.  The 
emanation  from  50  milligrams  of  radium  bromide  was 
collected  in  a  U-tube  by  driving  it  over  with  oxygen,  and 
then  condensed  in  the  tube  by  means  of  liquid  air.  It  was 
then  transferred  to  a  Pliicker  sparking  tube,  and  the  spec- 
trum taken.  At  first  there  were  no  helium  lines  present, 
but  a  new  spectrum,  presumably  that  of  the  emanation 
itself,  made  its  appearance.  In  a  jew  days  the  original 
spectrum  disappeared  and  the  spectrum  of  helium  came  out 
sharply. 

Thus  was  observed  for  the  first  time  in  the  history  oj  science 
the  formation  or  production  of  a  chemical  element.  Whether 
it  comes  directly  from  another  definite  chemical  element 
is  not  certain.  It  has  not  been  shown,  although  it  is  highly 
probable,  that  the  emanation  is  an  inert  chemical  element. 
It  is,  however,  certain  that  helium  is  thus  spontaneously 
produced  from  a  chemical  element  —  radium  —  as  one  of 
its  decomposition  products. 

THIS  IS  N0T  A  TRANSMUTATION  OF  THE  ELEMENTS 

Since  the  discovery  referred  to  above  was  made,  there 
has  been  so  much  written  about  the  "  Transmutation  of  the 
elements  having  been  effected,"  the  "Dream  of  the  alchemist 
realized,"  etc.,  that  a  word  of  warning  seems  highly 
desirable. 

From  some  of  the  statements  on  this  subject  that  have 
appeared,  any  one  unfamiliar  with  the  facts  might  con- 
clude that  we  are  now  able  to  effect  the  reciprocal  trans- 
formation of  practically  any  elementary  substances  almost 
ad  libitum.  We  are  no  more  able  to  effect  such  transjorma- 


134  THE  ELECTRICAL  NATURE  OF  MATTER 

tions  to-day  than  was  possible  a  thousand  years  ago,  nor 
has  such  a  transformation  ever  been  effected  by  any  one. 

It  appears  to  the  writer  to  be  one  thing  to  discover  an 
unstable  system  in  nature,  even  if  it  corresponds  to  our 
definition  of  chemical  element,  which  is  spontaneously 
undergoing  changes  that  are  largely  unaffected  even  by  the 
most  extreme  artificial  conditions  that  we  can  bring  to  bear 
upon  it,  and  giving  rise  to  another  elementary  substance 
as  one  of  its  decomposition  products;  and  an  entirely  differ- 
ent thing  to  effect  the  transformation  of  a  stable  element  into 
another  elementary  substance  by  purely  artificial  means. 

By  showing  that  helium  is  one  of  the  decomposition  prod- 
ucts of  radium,  it  has  been  shown  that  the  process  first  de- 
scribed does  actually  take  place,  at  least  in  the  case  of  one 
substance.  The  second  transformation  still  remains  to 
be  effected. 

In  calling  attention  to  the  above  distinction,  no  attempt 
is  made  to  belittle  the  magnificence  of  the  discovery  of  the 
spontaneous  formation  of  helium  from  radium,  which, 
when  we  consider  the  difficulties  involved  in  working  with 
such  small  quantities  of  substances,  is  to  be  placed  among 
the  great  achievements  of  modern  science,  and  could  not 
have  been  accomplished  by  a  man  of  less  experimental 
skill  than  that  possessed  by  Sir  William  Ramsay. 

FURTHER    EXPERIMENTS    ON    THE    PRODUCTION    OF    HELIUM 
FROM    RADIUM 

It  is  obvious  that  such  an  epoch-making  discovery  as 
that  described  above  would  be  subjected  to  the  closest 
scrutiny,  even  when  announced  by  such  a  distinguished 
authority  as  Ramsay.  The  first  question  that  would  occur 
to  any  one  is  this,  Could  the  helium  that  appeared  with  the 
emanation  have  been  occluded  in  the  radium  salt,  and  set 


HELIUM    PRODUCED    FROM    THE    EMANATION  135 

free  when  the  emanation  was  separated  from  the  salt? 
This  is,  of  course,  a  fair  question  to  ask,  but  the  answer 
was  furnished  by  Ramsay  himself.  The  salt  of  radium 
was  heated  in  contact  with  a  vacuum  pump  for  a  long  time, 
so  that  any  gas  occluded  in  the  radium  salt  must  have  been 
liberated.  When  the  salt  of  radium  thus  treated  was  allowed 
to  stand  until  the  emanation  was  formed,  and  this  emana- 
tion then  driven  off  and  collected  in  a  sparking  tube,  the 
presence  of  helium  lines  manifested  themselves  after  a  few 
days. 

One  fact,  as  has  doubtless  already  been  noted,  in  con- 
nection with  the  appearance  of  helium  lines  in  the  emana- 
tion, of  itself  argues  strongly  against  any  helium  having 
been  occluded  in  the  radium  salt,  and  then  set  free  when  the 
salt  was  dissolved  in  water.  The  emanation  freshly  dis- 
tilled from  the  radium  salt  showed  no  trace  of  the  helium 
spectrum.  The  spectrum  of  helium  appeared  only  after 
the  emanation  had  stood  for  some  time.  If  the  helium  was 
really  occluded  in  the  radium  salt,  its  spectrum  should 
have  manifested  itself  as  soon  as  it  was  driven  over  into  the 
Pliicker  tube.  The  fact  that  it  did  not,  but  appeared  after 
the  tube  containing  the  emanation  had  stood  for  a  time, 
is  a  strong  argument  in  favor  of  the  helium  having  been 
produced  by  some  change  taking  place  in  the  emanation 
itself. 

An  even  more  crucial  test,  if  possible,  of  the  occlusion  theory 
to  account  for  the  helium  was  made  by  Dewar,  Curie  and 
Deslandres.1  Four  hundred  milligrams  of  radium  bromide 
were  dried  and  placed  in  a  small  glass  vessel.  This  was 
connected  with  a  small  Geissler  tube,  and  the  whole  system 
exhausted.  The  degree  of  the  exhaustion  was  registered 
on  a  manometer.  During  the  three  months  that  the  bro- 

1  Compt.  rend.,  138,  190  (1904). 


136        THE  ELECTRICAL  NATURE  OF  MATTER 

mide  was  contained  in  the  exhausted  glass  vessel,  gas  was 
continually  being  given  off.  This  gas  was  found  spectro- 
scopically  to  be  hydrogen,  produced  probably  by  the  decom- 
position of  traces  of  moisture  in  the  salt  by  the  radium. 

The  radium  bromide  was  now  transferred  to  a  small 
quartz  vessel,  which  was  then  exhausted.  It  was  heated 
until  the  bromide  fused.  The  gases  that  were  given  off 
during  the  heating  were  passed  through  U- tubes  plunged 
in  liquid  air.  This  condensed  the  emanation  and  any  of 
the  less  volatile  gases.  During  the  heating  some  nitrogen 
gas  was  given  off,  having  been  occluded  in  the  salt.  The 
quartz  vessel  containing  the  radium  bromide,  from  which 
all  occluded  gases  had  now  obviously  been  removed,  was 
sealed  up  by  means  of  an  oxyhydrogen  blowpipe. 

After  the  tube  had  been  closed  twenty  days,  Deslandres 
studied  its  contents  spectroscopically.  The  gas  in  the  tube 
gave  now  the  entire  spectrum  of  helium. 

The  result  of  this  investigation  was  to  confirm  in  every 
respect  the  conclusion  previously  reached  by  Ramsay. 
Helium  is  formed  as  the  result  of  some  change  going  on  in 
the  radium,  or  in  the  emanation  from  the  radium. 

RELATION  BETWEEN  THE  EMANATION  AND  HELIUM 

Having  shown  that  the  helium  which  appeared  in  the 
sparking  tube  along  with  the  emanation  was  not  occluded 
in  the  salt  of  radium  from  which  the  emanation  was  obtained, 
the  next  question  that  was  raised  is,  What  relation  does  the 
helium  bear  to  the  emanation  from  which  it  is  produced? 
It  was  easy  to  show  in  a  number  of  ways  that  the  emanation 
itself  is  not  helium.  The  spectrum  that  first  appeared 
when  the  emanation  was  collected  in  the  sparking  tube 
was  not  that  of  helium  at  all,  but  was  an  entirely  new  spec- 
trum, not  corresponding  to  that  of  any  known  substance. 


HELIUM    PRODUCED    FROM    THE    EMANATION  137 

As  we  have  seen,  the  helium  lines  appeared  only  after  the 
emanation  had  stood  for  a  time.  Again,  the  emanation  is 
radioactive,  giving  off  a  particles.  Helium  does  not  give 
off  such  particles,  and,  indeed,  is  not  radioactive  at  all. 

Further,  the  emanation  is  condensed  by  passing  through 
a  tube  surrounded  by  liquid  air,  while  helium  can  be  con- 
densed to  a  liquid  only  below  the  temperature  of  liquid 
hydrogen  —  helium  liquifying  lower  than  any  other  known 
gas  —  at  about  —268°. 

Helium  is  the  lightest  gas  known  next  to  hydrogen,  its 
atomic  weight  being  four,  and  the  molecule  monatomic. 
The  emanation,  on  the  other  hand,  has  a  molecular  weight 
of  about  loo,  as  we  have  seen  from  diffusion  experiments. 

The  emanation  is,  then,  fundamentally  different  from 
helium  in  all  of  its  properties,  and  yet  helium  is  produced 
from  it  as  is  shown  by  spectrum  analysis.  A  theory  to 
account  for  the  production  of  helium  from  the  emanation 
should  be  mentioned,  even  if  it  is  insufficient,  as  it  will  be 
encountered  in  the  literature. 

It  has  been  suggested  that  radium  is  not  a  chemical  ele- 
ment, but  a  compound  of  helium  with  some  presumably 
unknown  element.  The  helium  that  was  produced  from 
radium  was  the  result  of  the  breaking  down  of  this  com- 
pound. There  are  a  number  of  reasons  why  this  theory  is 
insufficient.  In  the  first  place,  radium  has  all  the  proper- 
ties of  a  chemical  element  —  including  a  well-defined  spec- 
trum. Again,  such  a  theory  would  not  account  for  the 
radioactivity  of  radium,  nor  for  the  amount  of  heat  energy 
that  is  being  liberated  by  it. 

To  explain  the  properties  of  this  remarkable  substance* 
a  theory  along  entirely  new  lines,  as  we  shall  see,  is  neces- 
sary. 


138  THE   ELECTRICAL  NATURE   OF   MATTER 

SOME  REMARKABLE  RESULTS  OBTAINED  BY  THE  ACTION  OF 
THE   RADIUM  EMANATION 

Ramsay 1  has  found  that  when  a  salt  of  zirconium, 
thorium  or  bismuth,  or  hydrofluosilicic  acid  is  dissolved  in 
pure  water,  treated  with  the  radium  emanation  and  sealed 
up  in  a  glass  bulb,  carbon  dioxide  is  formed  in  the  solution. 

When  the  glass  bulb  is  allowed  to  stand  for  a  time  and 
then  opened,  from  one-tenth  to  one-half  of  a  cubic  centi- 
metre of  carbon  dioxide  was  pumped  out  of  the  solution. 

The  same  experiment  tried  with  a  large  number  of  other 
salts  gave  a  negative  result.  Blank  experiments  in  which 
no  salt  was  used  gave  negative  results  and  other  experi- 
ments in  which  no  radium  emanation  was  conducted  into 
the  solution  also  gave  no  carbon  dioxide. 

It  therefore  seems  to  be  pretty  well  established  that  in 
these  experiments  carbon  dioxide  is  formed. 

The  fundamental  question,  however,  is,  formed  from  what? 
Ramsay  thinks,  from  the  metal  of  the  salt  or  from  the  silicon 
of  the  hydrofluosilicic  acid,  by  these  elements  being  decom- 
posed by  the  radium  emanation.  He  points  out  that  thorium, 
zirconium,  and  silicon  all  fall  in  the  same  group  of  the 
Periodic  System  with  carbon  —  Group  IV;  and  that  bis- 
muth falls  in  the  next  group  —  Group  V.  He  thinks  that 
it  would  be  most  natural  that  these  elements  in  undergoing 
decomposition  would  break  down  into  simpler  elements  in 
the  same  periodic  group. 

This  conclusion  of  the  transformation  of  one  element  into 
another  may  strike  us  as  revolutionary  or  strange  even 
to-day.  A  dozen  years  ago  it  would  have  been  so  at  vari- 
ance with  all  of  the  facts  then  known  that  it  would  have 
met  only  with  opposition. 

1  Journ.  Chem.  Soc.  (1907-1909).     Chem.  Central  b.,  x,  1511  (1908). 


HELIUM  PRODUCED  FROM  THE  EMANATION  139 

The  unquestioned  fact,  however,  that  the  radium  emana- 
tion does  break  down  and  yield  a  chemical  element  helium 
pave  the  way  for  the  possibility  of  the  above  explanation 
offered  by  Ramsay.  Further,  the  study  of  the  radioactive 
elements  has  undoubtedly  shown  that  they  are  unstable, 
and  break  down  into  simpler  things.  Indeed,  the  existence 
of  radioactive  phenomena  depends  upon  this  very  fact. 
Since  the  radioactive  elements  are  unstable,  breaking  down 
spontaneously  into  simpler  things,  there  is  nothing  a  priori 
impossible  or  improbable  in  the  assumption  that  other  ele- 
ments, which  simply  represent  a  different  order  of  stability, 
may  be  broken  down  by  the  bombardment  of  the  radia- 
tions given  off  by  the  radium  emanation. 

All  attempts  to  explain  the  production  of  the  carbon 
dioxide  as  due  to  the  action  of  the  emanation  on  the  glass 
vessel  have  been  futile,  since  when  the  salt  is  omitted  from 
the  solution  no  carbon  dioxide  is  found;  and  the  salt  in 
question  would  have  nothing  to  do  with  the  action  of  the 
emanation  on  the  glass. 

As  no  other  satisfactory  explanation  of  the  origin  of  the 
carbon  dioxide  found  by  Ramsay  has  yet  been  furnished, 
it  seems  well  to  suspend  judgment  for  a  time,  remembering 
that  the  suggestion  of  Ramsay  is  now  quite  within  the 
range  of  possibility. 


CHAPTER  XIV 

INDUCED  RADIOACTIVITY 

IT  was  discovered  by  the  Curies  *  that  substances  in 
general,  that  are  left  for  some  time  in  the  presence  of  radium 
salts,  became  radioactive.  This  was  the  case  when  the 
substances  in  question  were  protected  from  any  dust  of 
the  radium  salt.  This  phenomenon  was  named  by  the 
Curies  Induced  radioactivity. 

This  property  of  rendering  substances  in  the  neighbor- 
hood radioactive  is  not  limited  to  radium.  Rutherford2 
found  that  salts  of  thorium  have  the  same  property,  and 
Debierne  showed  that  actinium  had  the  power  of  inducing 
radioactivity  to  a  very  high  degree. 

The  Curies 3  studied  this  property  of  radioactive  sub- 
stances in  the  following  manner.  They  used  a  closed 
vessel  into  which  the  radioactive  substance,  and  the  sub- 
stances on  which  radioactivity  was  to  be  induced,  were 
placed.  Under  these  conditions,  as  would  be  expected, 
more  marked  effects  were  produced  as  well  as  more  regular 
results  obtained. 

The  active  substance  was  placed  in  a  small  glass  vessel 
open  at  the  top,  which  was  suspended  in  the  centre  of  an 
inclosed  space.  Pieces  of  such  widely  different  substances 
as  glass,  hard  rubber,  paraffin  wax,  aluminium,  copper 
and  lead,  having,  however,  equal  surfaces,  were  inclosed 

1  Ann.  Chim.  Phys.  [7],  30,  289  (1903). 
2 Phil.  Mag.,  49,  161  (1900). 
3  Ann.  Chim.  Phys.  [7],  30,  291  (1903). 
140 


INDUCED   RADIOACTIVITY  141 

in  the  space  along  with  the  vessel  containing  the  radium 
salt.  It  was  found  that  all  of  these  substances  became  radio- 
active, and  were  radioactive  to  just  exactly  the  same  extent 
when  the  free  surfaces  were  the  same. 

To  test  whether  the  induced  radioactivity  was  caused  by 
the  radiations  falling  directly  upon  the  plate,  a  thick  lead 
screen  was  placed  in  the  inclosed  space,  to  one  side  of  the 
vessel  containing  the  radium  salt;  and  behind  this  screen 
was  placed  a  piece  of  metal,  having  the  same  surface  area 
as  the  other  objects  in  the  inclosed  space.  It  was  found 
that  this  substance,  thus  protected  from  the  radiations 
given  out  by  the  active  salt,  became  just  as  radioactive  as 
the  other  substances  having  the  same  surface,  exposed  to 
the  direct  action  of  the  radiations. 

The  further  interesting  experiment  was  tried,  of  closing 
the  vessel  containing  the  radioactive  substance.  When 
the  vessel  was  closed  it  was  found  that  none  of  the  sub- 
stances became  radioactive.  By  closing  the  vessel,  then, 
the  power  of  the  radioactive  substance  to  induce  radio- 
activity in  other  bodies  was  lost.  It  was  found  that 
the  induced  activity  was  more  intense  and  more  regular 
if  the  active  substance  was  in  solution  than  if  it  was  in  the 
solid  state. 

Water  itself  becomes  radioactive  if  allowed  to  stand  in  a 
closed  vessel  along  with  some  salt  of  radium. 

Certain  substances,  such  as  glass,  paper,  and  especially 
zinc  sulphide,  became  phosphorescent  under  the  same  con- 
ditions as  those  to  which  the  above-named  objects  were 
subjected.  When  the  induced  radioactivity  of  these  phos- 
phorescent substances  is  measured,  it  is  found  to  be  the 
same  as  the  induced  radioactivity  of  other  non-phosphores- 
cent substances,  subjected  to  the  same  exciting  cause. 
The  production  of  phosphorescence  in  such  bodies,  then, 


1^2  THE  ELECTRICAL  NATURE  OF  MATTER 

neither  increases  nor  diminishes  the  excited  radioactivity 
produced  in  them. 

The  Curies  also  established  the  following  facts.  If  a 
given  object  is  exposed  to  a  radioactive  body  in  a  closed 
vessel,  the  induced  radioactivity  in  the  object  increases 
with  the  time  of  the  exposure,  until  a  certain  definite,  maxi- 
mum value  is  reached.  This  maximum  value  is  indepen- 
dent of  the  nature  of  the  gas  that  fills  the  vessel  containing 
the  radioactive  substance,  and  the  material  on  which  radio- 
activity is  to  be  induced;  and  is  dependent,  for  a  given 
arrangement  of  the  apparatus,  only  upon  the  amount  of 
the  radioactive  substance  present  in  solution  in  the  con- 
fined space. 

INDUCED  RADIOACTIVITY  PRODUCED   BY  DECOMPOSITION 
PRODUCTS   OF  THE   EMANATION 

It  has  already  been  shown  that  the  induced  activity  is 
not  due  to  the  radiations  from  the  radioactive  bodies,  since, 
when  the  radiations  are  cut  off  from  an  object  by  a  thick 
lead  screen,  this  object  still  becomes  radioactive  if  contained 
in  a  vessel  along  with  the  radioactive  substance.  It  has  also 
been  shown  that  if  the  vessel  containing  the  radioactive 
material  is  completely  closed,  the  radioactive  substance  in 
the  vessel  does  not  excite  radioactivity  in  objects  around  it. 

This  would  strongly  indicate  that  the  excitant  of  radio- 
activity must  be  something  analogous  in  properties  to  a 
gas,  since  it  is  so  readily  cut  off,  and  since  it  can  pass  around 
a  screen  and  induce  radioactivity  in  an  object  placed  be- 
hind the  screen  just  as  if  the  screen  was  not  present. 

The  only  substance  given  off  from  such  radioactive  bodies 
as  thorium,  actinium,  and  radium,  which  has  the  properties 
of  a  gas,  is  the  emanation ;  and  we  should  expect  that  the  in- 
duced or  excited  radioactivity  was  caused  by  the  emanation. 


INDUCED  RADIOACTIVITY  143 

This  supposition  was  greatly  strengthened  by  the  fact 
that  only  those  elements  that  produce  an  emanation  have 
the  power  of  exciting  radioactivity  in  non- radioactive  sub- 
stances. 

This  view  that  the  induced  radioactivity  was  caused  by 
the  emanation  we  owe  to  Rutherford,  who  furnished  a 
number  of  lines  of  evidence  for  his  theory.  He  showed 
that  when  the  emanation  from  radium  was  cut  off,  the 
radium  lost  its  power  of  inducing  radioactivity  in  other 
bodies.  He  also  showed  that  the  induced  radioactivity  was 
proportional  to  the  emanating  power  of  the  substance  induc- 
ing it.  The  amount  of  the  emanation  present  was  measured 
by  its  power  to  ionize  a  gas  and  thus  render  it  a  conductor. 
When  this  was  compared  at  different  intervals  with  the 
radioactivity  produced,  it  was  found  that  the  two  are  pro- 
portional, to  within  the  limits  of  experimental  error. 

INDUCED  RADIOACTIVITY  UNDERGOES  DECAY 

We  have  seen  that  the  radioactivity  of  the  emanation 
itself  undergoes  decay.  Since  the  emanation  is  the  cause 
of  the  induced  radioactivity,  we  should  naturally  expect 
that  the  induced  radioactivity  itself  would  decay  with  time, 
and  such  is  the  fact. 

If  a  body  is  subjected  for  a  considerable  time  to  the 
emanation  from  thorium,  and  then  removed,  the  excited 
radioactivity  decays  regularly,  reaching  half  of  its  initial 
value,  according  to  Rutherford,  in  about  eleven  hours. 
The  rate  of  the  decay  of  the  inducted  radioactivity,  like 
so  many  other  properties  of  radioactive  substances,  is 
apparently  independent  of  many  of  the  conditions  to  which 
it  is  subjected. 

The  induced  radioactivity  produced  by  the  emanation 
from  radium  decayed  much  more  rapidly  than  that  pro- 


144  THE  ELECTRICAL  NATURE  OF  MATTER 

duced  by  the  emanation  from  thorium.  It  undergoes 
changes  somewhat  analogous  to  those  already  considered, 
decaying  to  half-value  in  a  few  minutes. 

We  should  naturally  like  to  know  whether  this  decay  con- 
tinues to  the  limit.  Do  the  bodies  once  rendered  radioactive 
by  the  emanation  from  naturally  radioactive  substances 
quickly  become  completely  non- radioactive  again  ?  This  in- 
formation has  been  furnished,  at  least  in  part,  by  the  Curies. 
They  found  that  the  induced  activity  produced  by  radium 
diminished  to  half-value  in  a  few  minutes,  but  a  small, 
residual  activity  persisted  for  almost  an  indefinite  time. 

INDUCED    RADIOACTIVITY    DUE    TO    THE    DEPOSIT    OF    RADIO- 
ACTIVE MATTER 

The  relation  between  induced  radioactivity  and  the 
emanation  from  radioactive  substances  has  been  developed, 
and  it  has  been  shown  that  the  latter  is  the  £ause  of  the 
former.  This,  however,  but  raises  the  question,  how  does 
the  emanation  render  objects  exposed  to  it  temporarily 
radioactive?  To  answer  this  question  we  must  study 
closely  the  property  of  induced  radioactivity.  If  a  thorium 
or  radium  salt  is  placed  in  a  closed  vessel,  all  objects  in  the 
same  vessel,  whatever  their  nature,  are  rendered  radio- 
active. If,  however,  a  negative  electrode  is  introduced 
into  the  vessel,  all  the  excited  radioactivity  is  confined  to 
this  electrode.  A  convenient  method  of  performing  this 
experiment  is  to  introduce  the  radioactive  salt  into  a  metal 
vessel  which  is  connected  with  the  positive  pole  of  a  battery. 
A  metal  wire  is  introduced  into  the  middle  of  the  vessel, 
passing  through  an  insulating  stopper.  This  wire  is  made 
the  negative  pole  of  the  battery.  Under  these  conditions, 
the  wire  is  the  only  object  in  the  vessel  that  is  rendered 
radioactive,  and  its  induced  radioactivity  may,  according 


INDUCED  RADIOACTIVITY  145 

to  Rutherford,  become  many  thousand  times  greater  than 
the  natural  activity  of  the  thorium  salt  which  induced  the 
activity  in  the  negative  electrode. 

It  is  difficult  to  account  for  this  fact,  together  with  the 
fact  that  the  emanation  is  the  cause  of  all  the  induced  radio- 
activity, on  any  other  ground  than  that  the  induced  radio- 
activity is  produced  by  a  deposit  of  radioactive  matter  upon 
objects  placed  in  the  neighborhood  of  naturally  radioactive 
substances. 

This  theory  would  explain  the  above  and  correlated  facts. 

To  propose  a  theory  that  explains  all  the  known  facts 
and  predicts  new  ones  is  one  thing,  but  to  show  that  this 
is  the  only  suggestion  that  will  account  for  these  facts  is 
quite  a  different  matter.  Further,  the  value  of  a  theory  or 
generalization  is  to  be  tested  rather  by  its  ability  to  predict 
new  and  undiscovered  facts,  and  then  have  the  predictions 
verified  by  experiments,  than  simply  to  account  for  what 
is  already  known. 

If  the  induced  radioactivity  is  due  to  the  deposition  of 
radioactive  matter  upon  non-radioactive  substances,  then 
this  matter  would  have  definite  properties.  We  ought  to 
be  able  to  remove  it  mechanically  from  the  object  upon 
which  it  was  deposited,  etc. 

We  shall  now  see  that  the  cause  of  the  induced  radio- 
activity can  be  removed  mechanically  and  otherwise  from 
objects  upon  which  it  has  been  deposited,  and  that  its 
properties  have  already  been  studied  in  some  detail. 

PROPERTIES    OF    THE    RADIOACTIVE    MATTER    DEPOSITED    BY 
THE  EMANATION  FROM  RADIOACTIVE  SUBSTANCES 

The  properties  of  the  active  matter  deposited  by  the 
emanation  from  thorium  have  been  studied  by  von  Lerch  * 

1  Ann.  d.  Phys.,  12,  745  (1903). 


146        THE  ELECTRICAL  NATURE  OF  MATTER 

and  by  Rutherford.1  The  active  matter  was  allowed  to 
deposit  upon  platinum,  and  its  solubility  in  different  sol- 
vents then  determined  by  measuring  the  decrease  in  the 
induced  radioactivity  of  the  platinum.  This  active  matter 
was  insoluble  in  such  organic  solvents  as  ether  and  alcohol. 
It  was  dissolved  by  hydrochloric  acid,  and  the  solution  was 
radioactive.  The  radioactivity  of  this  solution  was  greatly 
diminished  by  causing  a  precipitate  to  be  thrown  down 
from  it.  Thus,  if  barium  chloride  was  added  to  the  hy- 
drochloric acid  solution,  and  the  barium  thrown  down  as 
sulphate,  most  of  the  radioactive  matter  was  carried  down 
by  the  precipitate  which  was  strongly  radioactive. 

If  a  piece  of  magnesium  was  exposed  to  the  emanation 
from  thorium  until  it  became  highly  radioactive,  and  then 
dissolved  in  hydrochloric  acid,  the  magnesium  when  pre- 
cipitated as  carbonate  or  phosphate  carried  down  with  it 
the  radioactive  matter. 

Rutherford  showed  that  the  active  matter  can  be  removed 
from  an  object  upon  which  it  has  been  deposited,  purely 
mechanically.  If  a  piece  of  platinum  wire  has  been  ren- 
dered highly  radioactive  by  exposing  it  for  some  time  to 
the  emanation  from  thorium,  and  is  then  rubbed  with  a 
piece  of  sand-paper,  nearly  all  of  the  radioactive  matter 
can  be  removed  from  the  platinum.  The  sand-paper,  in 
turn,  becomes  radioactive. 

We  know  less  about  the  properties  of  the  radioactive 
matter  deposited  by  the  emanation  from  radium,  since,  as 
we  have  seen,  this  decays  much  more  rapidly  than  the  de- 
posits from  the  emanation  given  off  by  salts  of  thorium. 
It  has,  however,  been  shown  that  the  radioactive  matter 
from  radium  differs  at  least  in  its  solubility  from  the  radio- 
active matter  deposited  from  thorium. 

iPhys.  Zeit.,3,  254(1902). 


INDUCED  RADIOACTIVITY  147 

EMANATION  X 

The  above  facts  suffice  to  show  that  induced  radioactivity 
is  caused  by  the  deposit  upon  the  non-radioactive  substance 
oj  a  radioactive  form  of  matter,  which  can  be  removed  from 
the  substance  either  mechanically  or  chemically,  and  which 
has  definite  chemical  and  physical  properties  of  its  own. 
This  radioactive  deposit  has  been  termed  by  Rutherford 
emanation  X.  Another  name  was  given  to  it  later,  as  we 
shall  see.  The  amount  of  such  radioactive  matter  that  is 
deposited  is  extremely  small.  This  is  what  we  should 
expect,  since  we  have  learned  that  the  amount  of  the  emana- 
tion itself,  given  off  by  the  most  active  substances,  radium 
and  actinium,  is  almost  infinitesimal.  Rutherford  points 
out  that  no  matter  how  long  a  piece  of  metal  is  exposed  to 
the  emanation,  the  amount  of  radioactive  matter  deposited 
is  too  small  to  be  detected,  even  with  the  most  refined  balance. 

Here  is  then  another  remarkable  fact  added  to  that  long 
list  of  such  facts  that  have  been  brought  to  light  as  the 
result  of  the  disco  very  and  study  of  radioactive  phenomena. 
Certain  radioactive  substances  give  off  almost  an  infinitesi- 
mal quantity  of  a  kind  of  matter  that  is  analogous  to  a 
gas,  and  which  has  properties  literally  undreamed  of  by  men 
of  science.  This  minute  quantity  of  substance  manifests 
most  of  the  radioactivity  shown  by  the  natural  radio- 
active substance  from  which  it  came.  It  produces  a  chemi- 
cal element  helium  as  one  of  its  decomposition  products, 
and  perhaps  most  remarkable  of  all  is  the  amount  oj  energy 
that  it  can  give  out  in  the  form  of  heat.  It  was  justly  re- 
garded as  one  of  the  most  surprising  facts  known  to  science, 
when  the  Curies  discovered  that  salts  of  radium  themselves 
gave  out  heat  in  such  quantity  that  a  piece  of  radium  would 
melt  its  own  weight  of  ice  every  hour. 


148        THE  ELECTRICAL  NATURE  OF  MATTER 

This  discovery  dwindles  into  insignificance  in  comparison 
with  that  made  by  Rutherford,  that  about  three-fourths 
of  this  heat  comes  from  something  that  exists  in  the  radium 
salt  in  relatively  infinitesimal  quantity,  and  which  is  con- 
tinually decaying  and  being  manufactured  by  the  radium 
—  the  emanation.  It  is  perhaps  no  great  cause  for  wonder 
that  such  a  discovery  should  have  raised  questions  even 
in  connection  with  such  a  fundamental  generalization  as 
that  of  the  conservation  of  energy. 

We  now  find  that  the  emanation  in  decaying  yields  a 
product,  which  must  exist  in  still  smaller  quantity  than 
the  emanation  itself,  and  which  has  the  power  of  rendering 
inactive  substances  on  which  it  is  deposited  strongly  radio- 
active. This  induced  or  excited  radioactivity  as  it  is  termed 
also  undergoes  decay,  showing  that  the  radioactive  matter 
deposited  by  the  emanation  undergoes  still  further  changes. 

Some  of  these  have  already  been  studied  by  Rutherford. 

SOME  FACTS  THAT  MUST  BE  TAKEN  INTO  ACCOUNT  IN 
DEALING  WITH  THE  DECAY  OF  INDUCED  OR  EXCITED 
RADIOACTIVITY 

In  studying  the  transformations  that  are  undergone  by 
the  radioactive  matter  deposited  by  the  emanation,  let  us 
first  turn  to  the  facts  that  have  been  brought  to  light  by 
Rutherford,  and  clearly  stated  by  him  in  his  Bakerian  lec- 
ture 1  before  the  Royal  Society.2 

To  simplify  the  matter,  we  shall  deal  with  the  transfor- 
mations in  detail  only  in  the  case  of  radium.  The  induced 
radioactivity  produced  by  radium  undergoes  decay,  and 
at  the  same  time  gives  off  a,  /3,  and  y  particles.  If  we 
measure  the  decay  of  the  excited  radioactivity  by  means  of 

^hil.  Trans.,  A,  204,  169  (1904). 
2  See  also  Phil.  Mag.,  8,  636  (1904). 


INDUCED  RADIOACTIVITY  149 

the  a  rays,  we  obtain  a  different  result  from  that  arrived 
at  if  we  measure  the  decay  by  means  of  the  ft  or  y  rays. 
The  decay  was  measured  by  means  of  the  a  rays,  also  by 
means  of  the  ft  rays,  and  finally  by  means  of  the  y  rays. 

Some  of  the  results  that  were  obtained  in  the  case  0}  radium 
are  the  following:  After  the  rod  was  exposed  to  the  radium 
emanation,  the  activity  as  measured  by  the  a  radiation 
decreased  at  first  comparatively  rapidly.  The  decay  then  pro- 
gressed slowly,  finally  becoming  almost  zero. 

If  the  rate  of  decay  of  the  induced  activity  is  measured  by 
the  ft  rays,  quite  different  results  are  obtained.  The  decay  as 
measured  by  the  ft  radiation  after  the  first  ten  or  fifteen  min- 
utes, resembles  the  decay  as  measured  by  the  a  radiation. 
The  decay  as  measured  by  the  a  radiation  diminishes  very 
rapidly  for  the  first  fifteen  minutes.  This  is  not  the  case 
when  the  rate  of  decay  is  measured  by  the  ft  radiation. 

When  the  rate  of  decay  is  measured  by  the  y  radiation, 
results  are  found  which  are  exactly  similar  to  those  obtained 
with  the  ft  radiations.  This  is  just  what  we  should  expect 
from  the  relation  that  we  have  already  learned  exists  between 
the  ft  and  y  rays. 

INTERPRETATION  OF  THESE  FACTS 

The  following  interpretation  of  the  above  facts  has  been 
given  chiefly  by  Rutherford  *:  The  rapid  initial  decrease  in 
the  radioactivity,  as  measured  by  the  a  rays,  is  due  to  a 
change  taking  place  that  gives  rise  to  the  a  rays.  If  we 
examine  the  activity  as  measured  by  the  ft  ray  during  this 
period,  we  find  the  absence  of  any  sudden  drop  during 
this  initial  time.  This  shows  that  the  first  transforma- 
tion, which  takes  place  largely  during  the  first  three 
minutes,  does  not  give  out  ft  rays,  otherwise  the  activity 

1  Trans.  Roy.  Soc.,  204,  A,  169  (1904). 


150  THE   ELECTRICAL  NATURE   OF   MATTER 

as  measured  by  the  ft  rays  would  decrease  rapidly  during 
this  period. 

We  will  term  the  active  matter  deposited  by  the  emana- 
tion, not  emanation  X,  as  we  have  hitherto  done,  but 
radium  A.  Radium  A  gives  out  a  particles  only,  and 
quickly  undergoes  a  transformation  into  radium  B. 

A  study  of  the  rate  of  decay,  as  measured  by  a,  ft,  and  y 
rays  respectively,  leads  to  the  conclusion  that  a  second 
transformation  goes  on,  in  which  ft  and  7  radiations  are 
given  out.  In  this  second  change,  radium  B  passes  over 
into  radium  C.  The  time  required  for  radium  B  to  undergo 
half -decay  is  26.8  minutes,  and  radium  C  half -decays  in 
19.5  minutes,  as  shown  by  Bronson.1  Schmidt 2  showed 
that  radium  B  gives  a  slow  ft  and  a  7  radiation. 

Radium  C  has  been  separated  by  von  Lerch 3  electrolyt- 
ically  from  a  solution  containing  radium  B  and  radium  C. 
The  radium  B  was  then  precipitated  by  means  of  barium 
sulphate.  Of  the  different  transformation  products  of 
radium,  radium  C  is  one  of  the  elements  that  gives  out  7 
rays.  Further,  radium  C  gives  out  a  rays  with  a  greater 
velocity  than  any  other  known  substance.  While  the  a 
rays  from  radium  itself  can  penetrate  only  about  3^  cen- 
timetres of  air,  those  from  radium  C  can  traverse  about 
7  centimetres  of  air. 

While  the  activity  induced  by  radium  almost  vanishes 
in  a  day,  yet  there  remains  a  slight  " residual  activity," 
as  was  found  by  Madame  Curie.4  This  activity  shows  a 
and  ft  radiations,  but  instead  of  these  decreasing,  they  both 
increase  until  they  become  practically  constant.  Ruther- 
ford explains  these  facts  by  assuming  that  radium  C  passes 

1  Amer.  Journ.  Sci.  [4],  20,  55  (1905). 

2  Physik.  Zeit.,  6,  897  (1905). 

3  Sitzun.  Wien.,  115,  Ha,  197  (1906). 

4  Ann.  Chim.  Phys.  [7],  30,  289  (1903). 


INDUCED  RADIOACTIVITY  151 

over  into  radium  D,  which  has  a  very  long  life-history, 
half -decay  requiring  16.5  years.  D  giving  out  B  particles 
passes  over  slowly  into  radium  E. 

Rutherford  1  has  shown  that  radium  E  gives  out  ft  and 
possibly  7  rays  and  passes  over  into  radium  F.  This  is 
deposited  on  a  plate  of  bismuth  immersed  in  a  solution  of 
the  active  deposit.  Radium  F  gives  off  only  a  particles 
and  its  activity  decreases  to  half- value  in  about  136  days. 

Radium  F  is  much  more  active  than  pure  radium.  It 
has  been  shown  by  Rutherford  to  be  about  3,200  times  as 
radioactive  as  radium  at  its  minimum  activity,  or  800 
times  as  radioactive  as  normal  radium. 

We  must,  however,  go  one  step  farther  and  ask  the 
question,  what  becomes  of  radium  F?  Does  it  undergo 
still  further  transformation,  and  if  so,  into  what?  Ruther- 
ford has  thrown  light  indirectly  on  this  question.  We 
have  no  evidence  that  radium  F  passes  into  anything 
that  is  radioactive.  Radium  apparently  yields  four  sub- 
stances that  send  off  a  particles  —  radium  itself,  the  ema- 
nation, radium  A,  radium  C,  and  radium  F.  It  has  been 
regarded  as  highly  probable  that  the  a  particle  is  a  charged 
helium  atom.  It  would  then  have  a  mass  of  four,  and  five 
such  particles  a  mass  of  20.  If  the  atomic  weight  of  radium 
is  226,  the  end  product  formed  from  radium  F  would  have 
an  atomic  mass  of  206.  The  atomic  weight  of  lead  is  207.1. 
Lead  occurs  in  radioactive  minerals  in  quantities  propor- 
tional to  the  uranium  and  to  the  radium.  In  recent  tables 
of  transformations,  however,  radio-lead  is  placed  directly 
after  radium  D  as  on  page  199. 

The  second  line  of  argument  based  upon  the  presence 
of  lead  in  all  uranium  minerals  and,  therefore,  in  all  min- 
erals that  contain  radium,  is  fairly  convincing.     Recent 
1Phil.  Mag.,  10,  200  (1905). 


152  THE   ELECTRICAL  NATURE   OF  MATTER 

analyses  of  uranium  minerals  confirm  the  relation  pointed 
out  by  Rutherford. 

Other  elements,  such  as  hydrogen,  argon,  barium,  bis- 
muth, and  thorium,  occur  frequently  in  radioactive  minerals, 
and  it  may  be  shown  that  some  of  these,  in  addition  to 
helium,  are  produced  by  the  disintegration  of  radium.  Up  to 
the  present,  however,  the  evidence  in  the  case  of  lead  is  the 
most  satisfactory,  but  it  cannot  yet  be  regarded  as  proved 
that  lead  is  the  final  decomposition  product  of  radium. 

RADIUM  F  PROBABLY  IDENTICAL  WITH  POLONIUM 

One  of  the  most  interesting  consequences  of  Ruther- 
ford's study  of  residual  activities  is  that  he  has  made  it 
highly  probable  that  radium  F  is  identical  with  polonium. 
Madame  Curie  has  shown  that  the  activity  of  polonium  is 
not  constant,  but  decreases  irregularly,  probably  because 
her  material  was  impure.  When  pure  polonium  was  pre- 
pared it  was  found  to  decay  exponentially.  The  period  of 
this  decay  has  been  determined  many  times  and  the  most 
reliable  observations  give  the  constant  for  half  decay  as 
136  days.  Rutherford  found,  as  we  saw  above,  143  days 
for  radium  F,  St.  Meyer  and  von  Schweidler  2  138  days, 
which  is  in  satisfactory  agreement  with  136.  The  identity 
of  the  two  substances  is  also  further  established  by  their 
electrochemical  behavior. 

SUMMARY  OF  THE  DECOMPOSITION  PRODUCTS  OF  RADIUM 

We  have  now  followed  the  transformations  of  radium 
through  a  number  of  stages,  the  more  important  of  which, 
it  will  be  recalled,  are  the  following: 

1.  Radium  gives  off  a  and  ft  particles  and  yields  the 
emanation. 

2.  The  emanation  gives  off  a  particles  and  yields  emana- 
tion X,  or  radium  A. 


INDUCED  RADIOACTIVITY  153 

3.  Radium  A  gives  off  a  particles  and  yields  radium  ft. 

4.  Radium  B  gives  off  ft  and  7  radiations  and  yields 
radium  Q  and  €2. 

5.  Radium  Ci  gives  off  a,  ft,  and  7  rays  and  yields 
radium  Cj,  which  gives  off  ft  rays  yielding  radium  D. 

6.  Radium  D  gives  off  ft  radiations  and  yields  radium  E. 

7.  Radium  E  gives  off  ft  and  possibly  7  rays  and  yields 
radium  F. 

8.  Radium  F  gives  off  a  particles. 

DECOMPOSITION  PRODUCTS  OF  OTHER  RADIOACTIVE 
SUBSTANCES 

In  a  manner  similar  to  the  above,  it  has  been  made  highly 
probable  that  the  emanation  from  thorium  gives  rise  to  a 
radioactive  deposit  —  thorium  A,  which  undergoes  at  least 
two  transformations,  giving  thorium  B  and  thorium  C. 

The  complete  transformations  of  thorium  into  thorium 
C  are  given  in  the  following  table: 

Name  Time  of  half  decay  Kind  of  rays 

Thorium  3  X  io10  years  a 

Mesothorium  (i  and  2}  5.5  years  (?)  ft,  y 

Radio  thorium  737  days  o 

Thorium  X  3.64  days  a,  ft 

Emanation  54  seconds  a 

Thorium  A  0.14  seconds  a 

Thorium  B  10.6  hours  ft 

Thorium  C  (i  and  2)  i  hour  a 

Thorium  D  3.1  minutes  ft,  y 


152  THE   ELECTRICAL   NATURE    OF   MATTER 

analyses  of  uranium  minerals  confirm  the  relation  pointed 
out  by  Rutherford. 

Other  elements,  such  as  hydrogen,  argon,  barium,  bis- 
muth, and  thorium,  occur  frequently  in  radioactive  minerals, 
and  it  may  be  shown  that  some  of  these,  in  addition  to 
helium,  are  produced  by  the  disintegration  of  radium.  Up  to 
the  present,  however,  the  evidence  in  the  case  of  lead  is  the 
most  satisfactory,  but  it  cannot  yet  be  regarded  as  proved 
that  lead  is  the  final  decomposition  product  of  radium. 

RADIUM   F   PROBABLY   IDENTICAL  WITH   POLONIUM 

One  of  the  most  interesting  consequences  of  Ruther- 
ford's study  of  residual  activities  is  that  he  has  made  it 
highly  probable  that  radium  F  is  identical  with  polonium. 
Madame  Curie  has  shown  that  the  activity  of  polonium  is 
not  constant,  but  decreases  irregularly,  probably  because 
her  material  was  impure.  When  pure  polonium  was  pre- 
pared it  was  found  to  decay  exponentially.  The  period  of 
this  decay  has  been  determined  many  times  and  the  most 
reliable  observations  give  the  constant  for  half  decay  as 
136  days.  Rutherford  found,  as  we  saw  above,  143  days 
for  radium  F,  St.  Meyer  and  von  Schweidler  2  138  days, 
which  is  in  satisfactory  agreement  with  136.  The  identity 
of  the  two  substances  is  also  further  established  by  their 
electrochemical  behavior. 

SUMMARY  OF   THE  DECOMPOSITION  PRODUCTS   OF  RADIUM 

We  have  now  followed  the  transformations  of  radium 
through  a  number  of  stages,  the  more  important  of  which, 
it  will  be  recalled,  are  the  following: 

1.  Radium  gives  off  a  and  /8  particles  and  yields  the 
emanation. 

2.  The  emanation  gives  off  a  particles  and  yields  emana- 
tion X,  or  radium  A. 


INDUCED  RADIOACTIVITY  153 

3.  Radium  A  gives  off  a  particles  and  yields  radium  ft. 

4.  Radium  B  gives  off  ft  and  7  radiations  and  yields 
radium  Q  and  C%. 

5.  Radium  Ci  gives  off  a,  ft,  and  7  rays  and  yields 
radium  Cj,  which  gives  off  ft  rays  yielding  radium  D. 

6.  Radium  D  gives  off  ft  radiations  and  yields  radium  E. 

7.  Radium  E  gives  off  ft  and  possibly  7  rays  and  yields 
radium  F. 

8.  Radium  F  gives  off  a  particles. 

DECOMPOSITION  PRODUCTS   OF   OTHER  RADIOACTIVE 
SUBSTANCES 

In  a  manner  similar  to  the  above,  it  has  been  made  highly 
probable  that  the  emanation  from  thorium  gives  rise  to  a 
radioactive  deposit  —  thorium  A,  which  undergoes  at  least 
two  transformations,  giving  thorium  B  and  thorium  C. 

The  complete  transformations  of  thorium  into  thorium 
C  are  given  in  the  following  table: 

Name  Time  of  half  decay  Kind  of  rays 

Thorium  3  X  io10  years  a 

Mesothorium  (i  and  2)  5.5  years  (?)  ft,  y 

t 
Radio  thorium  737  days  a 

Thorium  X  3.64  days  a,  ft 

* 

Emanation  54  seconds  a 

Thorium  A  0.14  seconds  a 

i 

Thorium  B  10.6  hours  ft 

Thorium  C  (i  and  2)  i  hour  a 

Thorium  D  3.1  minutes  ft,  y 


156  THE  ELECTRICAL  NATURE   OF  MATTER 

Sir  William  Ramsay  summarizes  the  results  that  he  has 
obtained  with  radiothorium  as  follows: 

The  emanation  given  off  by  radiothorium  is  identical 
with  that  given  off  by  salts  of  thorium.  The  quantity,  as 
we  have  seen,  is  infinitesimal  in  the  case  of  thorium  com- 
pared with  the  amount  given  off  by  radiothorium.  The 
conclusion  is  that  ordinary  thorium  probably  contains  a 
trace  of  radiothorium  to  which  it  owes  its  radioactivity. 
Ramsay  announces  that  he  has  already  succeeded  in  sepa- 
rating a  part  of  the  radioactivity  from  the  thorium,  by  add- 
ing to  the  thorium  salt  a  salt  of  barium,  and  then  adding 
sulphuric  acid.  A  part  of  the  radiothorium  is  probably 
brought  down  along  with  the  barium  salt. 

Analogous  to  the  decomposition  products  of  uranium, 
Ramsay  suggests  the  following  scheme  as  representing  the 
probable  decomposition  products  of  thorium. 

Inactive  thorium  —  radiothorium  —  thorium  X  —  em- 
anation —  thorium  A  —  thorium  B  —  ?  —  helium. 

There  seems  to  be  no  doubt,  according  to  Ramsay,  that 
the  helium  found  in  thorianite  is  produced  from  the  radio- 
thorium  present  in  that  mineral. 

Uranium  does  not  yield  an  emanation,  but  uranium  X 
apparently  breaks  down  at  once  into  the  final  product. 

Actinium  yields  an  emanation,  which  decomposes  in  at 
least  three  stages,  giving  actinium  A,  B,  and  C. 

DECOMPOSITION  PRODUCTS   OF  ACTINIUM 

The  complete  series  of  decomposition  products  in  the 
case  of  actinium  is  given  in  the  following  table: 


INDUCED  RADIOACTIVITY  157 

Name  Time  of  half  decay  Kind  of  rays 

Actinium  ? 

t 

Radioactinium  19.5  days  a,  ft 

* 
Actinium  X  10.5  days  a 

t 

Emanation  3.9  seconds  a 

t 

Actinium  A  0.002  seconds  a 

t 
Actinium  B  36  minutes  ft 

t 
Actinium  C  (?)  2.1  minutes  a 

t 
Actinium  D  3.4  minutes  ft,  y 

Actinium  X  was  obtained  by  Godlewski 1  and  in  the  fol- 
lowing manner: 

To  the  hydrochloric  acid  solution  of  the  actinium,  ammo- 
nia was  added.  A  reddish-brown  precipitate  was  formed 
which  was  probably  the  hydroxide.  The  filtrate  was  evapo- 
rated to  dryness,  and  the  ammonium  salts  driven  off  by 
ignition,  when  a  small  black  residue  remained,  which  be- 
came white  on  heating. 

This  residue  was  found  to  be  intensely  radioactive  as  com- 
pared with  the  actinium  from  which  it  was  separated.  The 
activity  was  found  to  decrease  slowly  with  time,  according 
to  an  exponential  law. 

The  actinium  from  which  the  intensely  radioactive  pro- 
duct had  been  separated,  was  at  first  almost  nonradioactive. 
It  recovered  its  radioactivity  with  time,  the  recovery  curve 
being  the  inverse  of  the  decay  curve  of  the  residue. 

It  will  be  seen  that  the  above  results  are  strictly  analogous 
to  those  obtained  with  uranium  and  thorium.  From  the 
1  Phil.  Mag.,  10,  35. 


158        THE  ELECTRICAL  NATURE  OF  MATTER 

analogy  to  thorium  X,  the  above,  highly  active  product 
was  termed  actinium  X,  and  assigned  the  symbol  AcX. 

The  radioactivity  of  actinium  X,  when  first  separated 
from  the  actinium,  was  more  than  one  hundred  times  as  great 
as  that  of  the  actinium  itself.  The  residue  obtained  by 
evaporating  the  filtrate,  as  above  described,  is  not  all  actin- 
ium X,  but  consists  chiefly  of  non-radioactive  material, 
which  is  probably  some  of  the  rare  earths. 

The  analogy  between  uranium,  thorium,  and  actinium  is, 
as  we  have  seen,  very  striking.  There  is,  however,  one 
marked  difference.  After  thorium  X  is  removed  there 
remains  in  the  thorium  a  residual  activity,  which  amounts 
to  about  twenty-five  per  cent,  of  the  total  radioactivity 
possessed  by  normal  thorium. 

After  actinium  X  is  removed  from  actinium,  the  activity 
of  the  remaining  actinium,  when  tested  immediately,  is 
only  about  five  per  cent,  of  what  it  is  in  the  normal  sub- 
stance. 

Godlewski  tried  to  remove  this  small  residual  activity 
by  repeatedly  precipitating  the  actinium  solution  with 
ammonia.  Eight  precipitations  were  made  in  seven  hours. 
The  residual  activity,  however,  still  remained.  This  was 
probably  due  to  the  presence  of  a  small  amount  of  actinium 
X,  which  could  not  be  separated  from  actinium.  The 
latter  when  freed  from  actinium  X  is  perfectly  non-radio- 
active. The  production  of  actinium  X  from  actinium  is, 
as  shown  in  the  table  on  page  157,  radioactinium  being  an 
intermediate  product. 

Actinium  X  was  shown  to  give  out  a  rays.  That  the 
ft  rays  come  directly  from  radioactinium,  and  not  from  the 
excited  activity  resulting  from  the  deposit  of  the  emana- 
tion, is- made  highly  probable  by  the  following  facts:  The 
curves  of  decay  of  the  activity  of  radioactinium  are  the 


INDUCED   RADIOACTIVITY  159 

same,  whether  the  activity  is  measured  by  the  a  or  the  ft 
rays.  It  is  also  pointed  out  that  the  activity  of  radio- 
actinium,  measured  by  the  ft  rays  directly  after  strong  heat- 
ing, which  would  remove  all  the  cause  of  excited  activity, 
has  a  large  value  even  at  the  beginning.  This  would  not  be 
the  case  if  the  ft  rays  came  from  the  excited  or  induced 
activity. 

The  problem  of  the  origin  of  the  emanation  in  actinium 
was  then  attacked.  It  will  be  remembered  that  the  thorium 
emanation  comes  from  thorium  X.  Does  the  actinium 
emanation  come  from  actinium  X  ?  This  question  can 
be  easily  answered. 

Remove  the  emanation  from  actinium  containing  actin- 
ium X,  and  test  the  amount  by  the  activity.  Then  remove 
the  emanation  from  an  equal  amount  of  actinium  from 
which  the  actinium  X  has  been  separated,  and  test  its 
activity.  The  result  is  very  satisfactory. 

The  actinium  from  which  actinium  X  has  been  separated 
gives  practically  no  emanation.  Further,  the  amount  of 
the  emanation  increases  as  the  amount  of  actinium  X  in- 
creases, and  decreases  at  the  same  rate  that  the  activity, 
or  as  actinium  X,  decreases. 

Godlewski  points  out  that  the  emanation  being  present 
only  when  actinium  X  is  present,  and  being  always  pro- 
portional to  the  amount  of  actinium  X,  it  must  be  the 
product  of  actinium  X. 

The  products  of  actinium  that  have  thus  far  been  shown 
to  exist  are  the  following.  Actinium  yields  radioactinium 
which  yields  actinium  X.  Actinium  X  gives  out  a  rays  and 
yields  the  actinium  emanation.  The  actinium  emanation 
gives  out  a  particles  and  produces  actinium  A.  Actinium  A 
yields  actinium  B,  the  change  giving  out  a  particles. 
Actinium  B  gives  out  ft  particles  and  yields  actinium  C, 


l6o       THE  ELECTRICAL  NATURE  OF  MATTER 

which  gives  off  a  particles  yielding  actinium  D  which  gives 
off  ft  and  7  rays. 

Godlewski  also  shows  that  the  ft  rays  from  actinium 
differ  from  the  ft  rays  from  other  radioactive  substances. 
In  the  first  place  they  are  completely  homogeneous,  and 
in  the  second,  have  less  than  half  the  penetrating  power 
of  the  ft  rays  emitted  by  other  radioactive  substances. 

He  also  shows  that  the  7  rays  from  actinium  have  only 
about  one-fourth  the  penetrating  power  of  the  7  rays  from 
radium. 

The  chemical  nature  of  such  products  as  those  just  de- 
scribed is  entirely  unknown,  and  will  remain  so  until  they 
can  be  obtained  in  sufficient  quantity  to  be  studied  at  least 
by  the  more  refined  chemical  methods. 

Recent  work  makes  it  probable  that  certain  transforma- 
tions which  were  formerly  regarded  as  rayless,  give  off  a 
"soft"  or  not  highly  penetrating  kind  of  beta  ray. 


CHAPTER  XV 

PRODUCTION  OF  RADIOACTIVE  MATTER 
CONTINUOUS  FORMATION  OF  RADIOACTIVE  MATTER  IN 

URANIUM 

WE  have  learned  that  thorium  and  radium  from  which 
the  emanation  has  been  removed  have  lost  most  of  their 
radioactivity.  We  have  seen  that  the  emanation  loses  its 
activity,  but  what  is  more  important  in  the  present  con- 
nection, the  deemanated  substance  regains  its  radioactivity 
on  standing.  Further,  when  all  the  emanation  has  been 
driven  out  from  a  radioactive  substance  and  the  deemanated 
body  has  regained  its  radioactivity  on  standing  for  a  time, 
more  of  the  emanation  can  then  be  removed  from  this 
same  material. 

These  facts  can  best  be  interpreted  by  assuming  that 
some  change  is  continuously  going  on  in  the  radioactive 
substances,  which  gives  rise  to  the  emanation  and  restores 
the  radioactivity. 

In  connection  with  the  changes  taking  place  in  radio- 
active substances  a  most  important  discovery  was  made 
by  Sir  William  Crookes.1  He  found  that  a  very  active 
constituent  could  be  separated  from  uranium  by  chemical 
means,  and  that  the  remaining  uranium  was  not  appreciably 
radioactive. 

If  to  a  solution  of  a  uranium  salt  a  solution  of  ammonium 
carbonate  is  added,  the  uranium  is  precipitated.  If  an 

1  Roy.  Soc.  Proceed.,  66,  409  (1900). 
161 


1 62         THE  ELECTRICAL  NATURE  OF  MATTER 

excess  of  the  ammonium  carbonate  is  added  the  precipitate 
dissolves  in  this  reagent.  There,  however,  remains  a  small, 
light  brown  residue  that  does  not  dissolve  when  an  excess 
of  ammonium  carbonate  is  added.  This  residue  can  readily 
be  filtered  off  from  the  solution,  and  was  found  to  be  highly 
radioactive,  as  compared  with  uranium  itself.  This  residue 
was  called  by  Crookes  uranium  X,  and  was  given  the  symbol 
UrX. 

RECOVERY  OF  ACTIVITY  BY  URANIUM,  AND  DECAY  OF  ACTIVITY 
IN  URANIUM  X 

The  uranium  from  which  the  uranium  X  was  thus  sepa- 
rated was  left  much  less  radioactive.  If  this  uranium 
was  laid  aside  for  a  time,  it  was  found  to  regain  its  original 
radioactivity. 

The  uranium  X,  on  the  other  hand,  gradually  became  less 
active,  until  after  a  few  weeks  its  radioactivity  was  only 
half  as  intense  as  when  it  was  first  precipitated. 

The  rate  at  which  uranium  X  loses  its  activity  has  been 
carefully  studied.  Similarly,  the  rate  at  which  the  ura- 
nium, from  which  the  uranium  X  has  been  separated,  re- 
gains its  activity,  has  been  measured. 

These  results  show  that  the  uranium  X  loses  its  radio- 
activity just  as  rapidly  as  the  uranium  regains  its  activity. 
In  a  word,  that  in  ordinary  uranium  we  have  uranium  X 
undergoing  decay  at  just  the  same  rate  that  it  is  being  jormed, 
and  the  condition  that  exists  is  one  of  equilibrium  between 
these  two  opposite  processes. 

The  rate  at  which  uranium  recovers  its  radioactivity  has 
been  found  to  be  entirely  independent  of  the  conditions  to 
which  it  is  subjected. 

The  rate  at  which  uranium  X  loses  its  activity  has  also 
been  found  to  be  entirely  independent  0}  all  conditions  both 


PRODUCTION  OF  RADIOACTIVE  MATTER  163 

physical  and  chemical.  It  is  unaffected  by  the  state  of  aggre- 
gation of  the  radioactive  matter,  by  the  presence  of  any 
chemical  reagent,  and  what  is  more  surprising,  by  the  tem- 
perature to  which  it  is  subjected. 

We  can  now  see  why  the  radioactivity  of  uranium  is 
constant.  It  represents,  as  mentioned  above,  a  condition 
of  equilibrium  between  the  two  oposite  processes  —  the 
continual  production  of  the  radioactive  uranium  X  at  a 
constant  rate,  unaffected  by  any  change  of  conditions;  and 
the  continual  decay  of  the  activity  of  the  uranium  X  at  a 
constant  rate,  also  independent  of  all  conditions.  As  both 
of  these  processes  go  on  at  a  constant  rate,  the  equilibrium 
between  the  two  represents  a  condition  where  there  is  a 
constant  amount  of  uranium  X  in  the  uranium,  and  hence 
a  constant  radioactivity. 

RADIATION  FROM  URANIUM  X 

One  other  matter  of  importance  and  interest  should  be 
mentioned  before  leaving  the  discussion  of  radium  X. 

A  peculiarity  in  connection  with  the  radioactivity  of 
uranium  has  already  been  pointed  out.  It  does  not  give 
off  an  emanation. 

The  radiation  from  uranium  X  consists  of  ft  and  7  rays. 
These,  as  will  be  remembered,  consist  of  negative  charges 
of  electricity  shot  off  with  a  velocity  nearly  equal  to  that 
of  light,  and  contain  no  matter  whatsoever.  In  a  word, 
they  are  cathode  rays.  The  radiation  from  uranium  X 
contains,  then,  no  matter,  but  only  electricity. 

The  recognition  of  this  fact  is  of  the  very  greatest  im- 
portance in  connection  with  the  study  of  the  relations  be- 
tween uranium  and  uranium  X.  The  electrical  method 
cannot  be  used  in  this  connection,  since  the  ft  rays  produce 
but  little  ionization  in  a  gas.  The  photographic  method 


1 64        THE  ELECTRICAL  NATURE  OF  MATTER 

must  be  employed.  The  neglect  to  take  the  above  fact 
into  account  has  led  to  some  confusion  in  the  literature  of 
this  subject.1 

The  facts  just  pointed  out  in  connection  with  the  radia- 
tions given  off  by  uranium,  on  the  one  hand,  and  uranium 
X,  on  the  other,  are  of  prime  importance  in  determining 
the  radioactive  products  that  are  formed  from  uranium. 
In  addition  to  uranium  X,  which  gives  off  ft  and  7  particles, 
being  formed  from  uranium,  there  must  also  be  produced 
another  radioactive  product  which  sends  off  a  particles. 

As  we  have  just  seen,  uranium  X,  or  the  active  constituent 
which  gives  out  /8  and  7  rays,  has  been  separated  from  ura- 
nium; but  the  other  active  product  which  gives  out  the  a 
radiations  has  not  yet  been  separated  by  any  means  from 
salts  of  uranium. 

CONTINUOUS     FORMATION    OF    RADIOACTIVE    MATTER    FROM 

THORIUM 

We  have  just  seen  that  Sir  William  Crookes  succeeded, 
by  purely  chemical  means,  in  separating  from  uranium  a 
radioactive  constituent  which  was  fundamentally  different 
from  uranium  itself. 

A  similar  separation  has  been  effected  by  Rutherford 
and  Soddy 2  in  the  case  of  thorium.  When  a  thorium  salt 
is  dissolved  in  water  and  the  solution  treated  with  ammonia, 
the  thorium  is  precipitated.  When  tested,  the  precipitate 
was  found  to  be  much  less  radioactive  than  the  thorium  salt. 
The  solution  from  which  the  thorium  had  been  precipitated 
by  ammonia  was,  after  filtering,  completely  evaporated, 
and  the  residue  highly  heated  to  remove  salts  of  ammonia. 

1  Soddy:  Journ.,  Chem.Soc.,  81,860  (1902);  Rutherford  and  Grier:  Phil. 
Mag.,  4,  315,  (1902). 

2  Journ.  Chem.  Soc.,  81,  837  (1902). 


PRODUCTION  OF  RADIOACTIVE  MATTER  165 

The  final  residue  after  heating  was  found  to  be  very 
radioactive.  Indeed,  in  some  cases,  more  than  a  thousand 
times  more  radioactive  than  the  thorium  salt  itself. 

The  highly  active  residue  was  very  small  in  quantity, 
and,  therefore,  must  have  contained  some  substance  whose 
radioactivity  was  very  intense.  This  product  obtained 
from  thorium  was  called  by  Rutherford  and  Soddy  thorium 
X,  and  assigned  the  symbol  ThX. 

This  substance  was  shown  to  be  soluble  in  water,  since 
when  thorium  oxide  is  shaken  with  water  the  radioactive 
constituent  is  partly  dissolved,  while  thorium  oxide  itself 
is  insoluble  in  water. 

If  a  solution  of  a  thorium  salt  is  treated  with  ammonium 
carbonate,  the  thorium  X  is  precipitated  along  with  the 
thorium.  The  method  of  separating  thorium  X  from 
thorium  is,  then,  very  different  from  that  required  to  separate 
uranium  X  from  uranium.  We  shall  now  study  some  of 
the  properties  of  thorium  X. 

PROPERTIES  OF  THORIUM  X  —  DECAY  OF  ITS  RADIOACTIVITY 

Thorium  X,  when  separated  from  thorium  by  the  method 
above  described,  is  highly  radioactive  as  we  have  seen. 
Its  radioactivity,  however,  decays,  having  only  about  half 
its  initial  value  after  3.64  days. 

The  rate  at  which  thorium  X  decays  or  loses  its  radio- 
activity, like  uranium  X,  is  uninfluenced  by  any  known 
physical  or  chemical  condition.  Moisture,  pressure,  and 
even  temperature  have  no  influence  on  the  rate  at  which 
thorium  X  decays. 

THORIUM  X  PRODUCES  THE  THORIUM  EMANATION 

Both  thorium  and  radium  are  capable  of  yielding  that 
remarkable  substance  or  substances  already  studied  —  the 


1 66        THE  ELECTRICAL  NATURE  OF  MATTER 

emanation.  In  the  case  of  thorium,  does  the  emanation 
come  from  the  thorium  directly,  or  from  thorium  X?  This 
has  been  answered  by  Rutherford  and  Soddy.1 

If  thorium  X  is  completely  removed  from  thorium  by 
repeated  precipitations,  the  thorium  has  no  appreciable 
power  to  give  off  the  emanation.  If,  however,  the  thorium 
is  allowed  to  stand  for  some  time,  it  can  give  off  an  appre- 
ciable quantity  of  the  emanation.  This  is  due,  as  we  shall 
learn,  to  the  production  of  thorium  X  which  is  going  on  in 
the  thorium  itself. 

The  thorium  X  when  first  separated  from  the  thorium 
has  marked  power  to  produce  the  thorium  emanation.  As 
the  thorium  X  decays  it  has  been  shown  that  its  power  to 
produce  the  emanation  becomes  less,  and,  indeed,  in  the 
same  ratio.  This  shows  .that  the  thorium  X  produces  the 
thorium  emanation. 

The  changes  that  are  going  on  in  thorium  can  now  be 
followed,  at  least  in  part.  The  thorium  atom  loses  an 
a  particle  producing  indirectly  thorium  X.  Thorium  X, 
like  thorium  itself,  is  an  unstable  system  and  changes  take 
place  in  it.  Thorium  X  loses  a  and  ft  particles,  and  the 
thorium  emanation  is  produced.  The  emanation  is  a  dif- 
ferent substance  from  thorium  X  from  which  it  was 
formed.  This  conclusion  is  confirmed  by  a  comparison  of 
all  of  the  properties  of  these  two  substances.  (See  p.  153.) 

Thorium  X  differs  from  uranium  X  in  the  kind  of  radiation 
given  out  by  it.  Thorium  X  gives  out  chiefly  a  particles, 
while  uranium  X,  as  we  have  seen,  gives  out  mainly  ft  rays. 

RECOVERY  OF  RADIOACTIVITY  BY  THORIUM 

We  have  become  familiar  with  a  method  for  separating 
thorium  X  from  thorium.  This  method  effects  almost 

1  Journ.  Chem.  Soc.,  81,  849  (1902). 


PRODUCTION  OF  RADIOACTIVE  MATTER  167 

complete  separation  if  the  process  is  repeated  a  few  times. 
If  the  thorium  precipitated  by  ammonia  is  dissolved  in 
nitric  acid,  and  then  again  precipitated,  and  the  process 
repeated  twice,  the  resulting  thoria  is  only  about  one  one- 
hundredth  as  radioactive  as  ordinary  thorium.  The  active 
thorium  X  has,  thus,  for  the  most  part,  been  separated  from 
the  thorium. 

If  now  this  comparatively  non-radioactive  thorium  is  set 
aside,  it  regains  its  radioactivity.  A  careful  study  of  the 
rate  at  which  thorium  recovers  its  radioactivity,  after  thorium 
X  has  been  removed  from  it,  has  been  made  by  Rutherford 
and  Soddy.1  They  found  that  the  thorium,  in  general, 
recovered  its  radioactivity  at  the  same  rate  that  the  sepa- 
rated thorium  X  lost  its  radioactivity. 

A  careful  comparison  was  made  of  the  rate  at  which 
thorium  X  decays  with  time,  and  the  rate  at  which  thorium, 
from  which  thorium  X  has  been  separated,  recovers  its 
radioactivity,  and  the  results  plotted  in  curves.  It  was 
found  that  the  one  loses  its  radioactivity  just  as  rapidly  as 
the  other  regains  its  radioactivity. 

This  can  best  be  interpreted  by  assuming  that  thorium  X 
is  continually  being  produced  by  the  thorium.  The  rate  of 
production  is  just  equal  to  the  rate  at  which  thorium  X 
decays,  and  this  gives  us  the  condition  of  equilibrium  that 
obtains  in  ordinary  thorium. 

From  the  thorium  which  has  regained  its  original  radio- 
activity —  and  this  requires  about  a  month  —  a  new  portion 
of  thorium  X  can  be  separated,  and  exactly  the  same  amount 
as  measured  by  its  radioactivity  that  was  obtained  originally. 
The  non-radioactive  thorium,  from  which  the  second  por- 
tion of  thorium  X  had  been  separated,  recovers  its  radio- 
activity again,  at  the  same  rate  that  it  did  initially.  When 

1  Journ.  Chem.  Soc.,  81,  840  (1902). 


1 68        THE  ELECTRICAL  NATURE  OF  MATTER 

the  condition  of  equilibrium  is  reached  again,  a  new  por- 
tion of  thorium  X  can  be  separated,  which  is  equal  to  that 
originally  obtained,  and  thus  the  process  goes  on  slowly, 
apparently  until  all  of  the  thorium  is  transformed  into 
thorium  X.  This  complete  transformation  would  prob- 
ably require  millions  of  years.1 

RATE    AT    WHICH    THORIUM    RECOVERS    RADIOACTIVITY 
INDEPENDENT  OF  CONDITIONS 

We  would  naturally  ask  whether  transformations  like 
those  we  have  just  been  considering  resemble  ordinary 
chemical  reactions,  or  are  something  fundamentally  different 
from  them?  To  throw  any  light  on  this  question  we  must 
study  the  two  sets  of  transformations,  and  see  what  re- 
semblances or  differences  make  their  appearance.  Chemical 
reactions  are,  in  general,  affected  by  the  physical  conditions 
of  the  substances  that  are  reacting  —  by  the  state  of  aggre- 
gation, whether  solid,  liquid,  gas,  or  solution;  by  the  pres- 
sure to  which  they  are  subjected,  and  especially  by  the 
temperature. 

The  rate  at  which  thorium  X  is  formed  from  thorium 
seems  to  be  entirely  independent  oj  all  these  conditions.  It 
does  not  seem  to  matter  to  what  conditions  we  subject  the 
thorium  from  which  thorium  X  has  been  separated,  we 
cannot  affect  in  any  way  the  rate  at  which  it  recovers  its 
lost  radioactivity,  which  is  the  same  as  to  say,  the  rate  at 
which  it  produces  thorium  X. 

There  is,  then,  at  least  this  one  fundamental  difference 
between  the  formation  of  thorium  X  from  thorium,  and 
ordinary  chemical  transformations  —  the  former  is  inde- 
pendent of  all  the  conditions  to  which  the  substances  are 
subjected,  including  great  changes  in  temperature. 

1  Journ.  Chem.  Soc.,  81,  844  (1902). 


PRODUCTION  OF  RADIOACTIVE  MATTER  169 

RADIUM  DOES  NOT  GIVE  RISE  TO  SUBSTANCES  CORRESPONDING 
TO  URANIUM  X  AND  THORIUM  X 

Radium  has  not  thus  far  been  shown  to  yield  any  sub- 
stance analogous  to  those  formed  by  uranium  and  thorium, 
which  we  have  just  been  studying.  It  does  not  form  any 
intermediate  product,  but  apparently  yields  the  emanation 
at  once. 


CHAPTER  XVI 
THEORETICAL  CONSIDERATIONS 

IMPORTANCE  OF  A  THEORY  OR  GENERALIZATION 

THE  chief  aim  of  scientific  investigation  is  not  the  dis- 
covery of  isolated  facts.  Indeed,  we  might  continue  to 
unearth  such  facts  for  an  indefinite  time,  in  any  branch  of 
natural  science,  and  it  is  a  question  whether  such  knowl- 
edge ought  to  be  dignified  with  the  name  of  science. 

The  highest  aim  of  scientific  investigation  is  to  reach  a 
theory  or  generalization,  which,  when  sufficiently  estab- 
lished, becomes  a  law.  This  may  or  may  not  be  an  ulti- 
mate truth,  probably  is  not,  but  may  be  as  near  to  it  as  the 
methods  at  present  at  our  disposal  are  capable  of  leading. 

It  may  be  asked,  how  do  we  arrive  at  generalizations  in 
science  ?  The  answer  is,  for  the  most  part  by  the  inductive 
method.  We  discover  fact  after  fact  and  then  coordinate 
and  correlate  these  individual  facts,  and  the  result  is  a 
generalization. 

It  may  then  be  said,  and  fairly,  that  the  discovery  of 
facts  is  highly  important,  indeed  essential  to  the  discovery 
of  generalization  or  law.  From  this  no  man  of  science 
will  dissent.  The  discovery  of  isolated  facts  bears  the  same 
relation  to  science  as  the  making  of  bricks  to  architecture. 
The  bricks  are  absolutely  essential  in  constructing  the  build- 
ing, but  they  are  not  the  end  or  aim  of  the  architect.  They 
are  simply  a  means  toward  the  end,  which  is  utility,  or 
beauty,  or  both.  Just  so  in  the  investigation  of  natural 

170 


THEORETICAL  CONSIDERATIONS  171 

phenomena;  we  must  study  the  isolated  facts;  they  are  the 
bricks  or  individual  units  of  which  science  is  made.  They 
are,  however,  not  science,  and  not  the  end  of  scientific 
investigation.  They  are  but  the  means  to  the  end.  The 
generalization  in  science  may  be  compared  with  the  finished 
edifice  in  architecture. 

We  have  now  studied  a  large  number  of  facts  pertaining 
to  radioactivity.  Some  of  these  are  of  a  striking  nature, 
and  arouse  deep  interest  when  considered  by  themselves. 
Their  real  importance  and  significance,  however,  comes 
out  when  we  consider  them  in  their  relations  to  other  jacts, 
and  especially  to  well-established  generalizations,  which 
we  now  accept  as  the  philosophy  of  the  physical  sciences. 

We  shall  next  attempt  to  coordinate  the  facts  of  radio- 
activity, and  see  what  generalizations  have  been  reached. 
We  shall  learn  that  new  light  has  been  thrown  on  the  nature 
of  what  we  call  in  chemistry  the  atom,  and  on  the  genesis 
of  matter,  by  the  study  of  various  phenomena  connected 
with  radioactivity. 

THE  MORE  IMPORTANT  FACTS  IN  CONNECTION  WITH  URANIUM 

Before  taking  up  the  generalizations  that  have  been 
reached,  a  brief  summary  of  the  facts  in  connection  with 
the  several  radioactive  elements  will  be  given,  by  way  of 
review,  since  it  is  these  facts  that  have  to  be  dealt  with 
primarily  by  any  theories  that  have  been,  or  may  be,  pro- 
posed. 

The  element  uranium  gives  off  a,  ft  and  y  rays.  The 
alpha  rays  are  composed  of  material  particles,  each  having 
a  mass  from  two  to  four  times  the  mass  of  the  hydrogen 
atom.  These  particles  are  shot  off  at  very  high  velocities, 
and,  therefore,  have  considerable  kinetic  energy.  The  a 
particles  are  charged,  and  are,  therefore,  deflected  in  a 


172  THE   ELECTRICAL  NATURE   OF   MATTER 

magnetic  field.  The  direction  of  their  deflection  shows 
that  they  are  charged  positively.  Every  a  particle  carries 
two  unit  electrical  charges,  as  we  say;  that  is,  twice  the 
amount  of  electricity  carried  by  a  univalent  ion  in  solution. 

The  a  particles  produce  strong  ionization  of  the  gas 
through  which  they  pass;  indeed,  most  of  the  ionization 
effected  by  radioactive  substances  is  due  to  the  a  particles. 

The  a  particles  have  marked  power  to  produce  phospho- 
rescence in  certain  substances,  especially  zinc  sulphide  and 
barium  platinocyanide.  The  phenomena  that  manifest  them- 
selves in  the  spinthariscope  are  due,  for  the  most  part,  to 
the  a  particles. 

The  a  rays  produce  but  little  effect  upon  a  photo- 
graphic plate,  and,  therefore,  the  photographic  method 
cannot  be  used  to  measure  the  intensity  of  this  kind  of 
radiation.  The  a  particles  being  material  in  nature  are 
easily  cut  off  by  matter.  They  cannot  pass  even  through 
very  thin  metallic  screens. 

The  ft  rays  are  closely  analogous  to  the  cathode  rays. 
The  mass  of  the  ft  particle  is  about  -jyV^  of  the  mass  of 
the  hydrogen  ion.  These  particles  are  shot  off  with  dif- 
ferent velocities,  but  all  with  very  high  speed,  indeed,  of  the 
order  of  magnitude  of  light.  The  mass  being  small,  the 
kinetic  energy  of  the  ft  particle  is  much  less  than  that  of 
the  a  particle,  notwithstanding  the  fact  that  the  ft  particle 
moves  with  greater  velocity. 

The  ft  particles  are  deflected  by  a  magnetic  field,  indeed 
much  more  strongly  than  the  a  particles.  They  are,  how- 
ever, deflected  in  the  opposite  direction  to  the  a  particles, 
and  have  a  negative  charge.  Every  ft  particle  is  a  unit 
negative  charge  of  electricity. 

These  particles  produce  comparatively  little  ionization 
in  the  gas  through  which  they  pass. 


THEORETICAL   CONSIDERATIONS  173 

They  have  comparatively  little  power  to  excite  phos- 
phorescence. The  ft  particles  have  some  effect  upon  a 
photographic  plate.  They  are  cut  off  by  metallic  screens 
of  any  considerable  thickness,  but  have  much  greater  pen- 
etrating power  than  the  a  rays. 

The  gamma  rays  are  identical  with  the  X-rays.  These 
rays  are  apparently  shot  off  as  pulses,  with  very  high  veloci- 
ties. The  7  rays  are  not  deflected  at  all  even  by  a  very 
intense  magnetic  field.  They  produce  comparatively  little 
ionization  in  gases,  and  have  but  little  power  to  excite 
phosphorescence.  They  have  very  marked  action  on  a 
photographic  plate.  They  have  great  penetrating  power; 
not  only  many  times  greater  power  to  penetrate  matter 
than  the  ft  rays,  but  even  greater  penetrating  power  than 
the  X-rays  themselves.  As  has  already  been  pointed  out, 
Rutherford  has  been  able  to  detect  the  7  rays  from  radium 
after  they  have  passed  through  a  foot  of  solid  steel.  Gamma 
rays  are  produced  by  the  ft  rays,  and  are  never  present 
without  them. 

Uranium  is  continually  but  very  slowly  undergoing  a 
transformation,  giving  rise  to  a  form  of  matter  that  differs 
in  properties  from  the  uranium  itself.  This  new  form  of 
matter  is  called  uranium  X.  The  rate  at  which  uranium  X 
is  formed  is  independent  of  all  physical  and  chemical  con- 
ditions. In  the  formation  of  uranium  X  from  uranium,  a 
particles  are  given  off. 

Uranium  X,  in  turn,  undergoes  decomposition,  giving 
off  ft  and  7  rays  during  the  process.  The  rate  of  the  de- 
composition is  also  independent  of  all  conditions  (p.  199). 

THE  MORE  IMPORTANT  FACTS  IN  CONNECTION  WITH  THORIUM 

The  facts  in  connection  with  thorium  are  more  numerous 
than  with  uranium.  Thorium,  like  uranium,  gives  off  a,  ft 


174       THE  ELECTRICAL  NATURE  OF  MATTER 

and  7  rays.  It  undergoes  a  continuous  transformation, 
yielding  a  new  form  of  radioactive  matter  known  as  thorium 
X,  at  the  same  time  setting  free  a  particles.  (See p.  153.) 

Thorium  X  gives  off  a  and  probably  ft  particles,  and 
yields  an  emanation  —  the  thorium  emanation  —  which  is  a 
gas.  This  emanation  gives  off  a  particles  and  forms  thorium 
A,  a  radioactive  solid  which  undergoes  still  further  decom- 
position in  three  stages.  In  the  first  stage  ft  rays  are  sent 
out,  in  the  second  a  rays,  while  in  the  third  ft  and  7  rays 
are  given  off. 

Since  thorium  gives  rise  to  an  emanation  which  decom- 
poses into  a  solid  form  of  matter  —  thorium  A  —  that  is 
radioactive  and  deposits  upon  other  objects,  thorium  is 
capable  of  inducing  or  exciting  radioactivity  in  bodies 
placed  in  its  neighborhood.  Thorium  thus  differs  strik- 
ingly from  uranium,  which  forms  no  emanation,  and  which, 
therefore,  cannot  induce  radioactivity  on  neutral  objects 
even  when  in  close  proximity  to  them. 

THE  MORE  IMPORTANT  FACTS  IN  CONNECTION  WITH  RADIUM 

The  best  studied  of  all  the  strongly  radioactive  substances, 
by  far,  is  radium.  Its  radioactivity  is  at  least  a  million  and 
a  half  times  that  of  metallic  uranium,  which  is  taken  as  the 
standard  unit. 

Radium  does  not  yield  any  radioactive  substance  anal- 
ogous to  uranium  X  or  thorium  X.  It  appears  to  produce 
the  emanation  at  once  from  itself,  giving  off  a  and  ft  par- 
ticles. The  emanation  gives  off  a  particles,  and  emanation 
X  or  radium  A  results.  This  undergoes  a  number  of 
transformations,  which  have  already  been  recognized. 
During  the  first  transformation  a  rays  are  sent  off;  during 
the  second  ft  and  7  rays  are  emitted,  while  during  the 
third  stage  of  the  transformation  a,  ft  and  7  rays  are  all 


THEORETICAL  CONSIDERATIONS  175 

given  out.  During  the  fourth  stage  of  the  transformation 
ft  radiations  are  given  off;  while  during  the  fifth  stage  ft 
and  y  rays  are  liberated.  Radium  possesses  a  number  of 
unique  properties,  all  of  which  are  very  remarkable.  Radium 
is  the  only  known  chemical  element  that  produces  of  itself 
another  chemical  element,  or  can  be  made  to  produce  such 
by  any  known  means.  Radium,  or  more  exactly,  the 
radium  emanation,  in  undergoing  decomposition  spontane- 
ously yields  the  element  helium.  This  discovery  was  so 
surprising  and  so  directly  at  variance  with  all  of  our  previous 
conceptions  of  a  chemical  element,  that  it  was  subjected  to 
the  severest  experimental  tests.  It  has  withstood  all  criti- 
cism, and  is  beyond  doubt  a  fact. 

A  number  of  the  other  properties  of  radium  are  scarcely 
second  in  importance  to  the  production  by  it  of  helium. 
Radium  has  the  power  of  charging  itself  electrically,  and 
it  is  the  only  substance  known  that  has  this  power. 

Radium  also  has  the  property  of  producing  light  or  be- 
coming self-luminous. 

Most  remarkable,  however,  is  the  amount  of  heat  that  is 
being  continuously  set  free  from  radium.  It  will  be  re- 
membered that  radium  produces  enough  heat  to  melt  its 
own  weight  of  ice  every  hour. 

When  we  consider  the  almost  limitless  time  over  which 
radium  can  thus  continue  to  give  out  heat,  we  see  how 
enormous  is  the  amount  of  energy  that  this  substance  can 
liberate.  It  is  of  a  magnitude  entirely  incomparable  with 
the  amount  of  heat  set  free  in  the  most  strongly  exothermic 
chemical  reactions.  The  enormous  magnitude  of  the 
energy  that  can  be  liberated  by  radium  must  be  classed  as 
one  of  the  most  important  discoveries  in  modern  science. 

With  the  facts  enumerated  above  at  our  command  we 
can  now  proceed  to  discuss  intelligently  the  generalizations 


176        THE  ELECTRICAL  NATURE  OF  MATTER 

and  conclusions  that  have  been  reached  as  the  result  of  the 
study  of  radioactivity. 

THEORY     OF    RUTHERFORD    AND     SODDY    TO     ACCOUNT    FOR 
RADIOACTIVE  PHENOMENA 

The  only  theory  thus  far  proposed,  which  accounts  at  all 
satisfactorily  for  the  phenomena  discovered  in  connection 
with  the  radioactive  elements,  and  which  will  probably 
prove  to  be  of  epoch-making  importance,  is  that  advanced 
by  Rutherford  and  Soddy.1  The  key  to  this  theory  is  that 
the  radioactive  elements  are  unstable.  The  atoms  of  these 
substances  represent  unstable  systems,  which  are  continually 
undergoing  rearrangement  and  decomposition.  A  definite 
number  of  the  atoms  of  any  radioactive  element  become 
unstable  in  any  given  time.  They  each  throw  off  an  a 
particle,  and  the  next  stage  results.  In  the  case  of  uranium 
and  thorium,  there  are  formed  indirectly  uranium  X  and 
thorium  X.  These  products  are  in  turn  unstable.  They 
throw  off  a  or  ft  particles,  and  in  the  case  of  thorium  an 
emanation  results.  The  radium  atom  throws  off  an  a 
particle  or  a  and  ft  particles,  and  yields  at  once  the  emana- 
tion. (See  p.  199.) 

The  emanation  also  is  in  an  unstable  condition.  It 
throws  off  a  particles  and  yields  a  radioactive  solid,  which, 
when  deposited  upon  non-radioactive  matter,  induces  in  it 
radioactivity.  This  solid,  or  emanation  X,  or  radium  A 
as  it  is  termed,  is  also  unstable  and  undergoes  further  trans- 
formations. In  the  case  of  radium  a  fairly  large  number 
of  steps  have  already  been  traced.  In  the  earlier  stages 
of  the  transformations  of  emanation  X  either  a  particles 
or  no  radiations  escape.  If  no  radiation  is  given  out  and 
we  still  have  a  well-marked  transformation  taking  place, 
1  Phil.  Mag.,  5,  576  (1903). 


THEORETICAL   CONSIDERATIONS  177 

it  probably  means  that  the  parts  of  the  atom  are  simply 
undergoing  rearrangement  without  losing  any  constituent. 

The  ft  and  7  rays  are  given  off  chiefly  in  the  later  stages 
of  the  decompositions  that  are  taking  place  in  the  radioac- 
tive atoms. 

These  unstable  atoms,  which  are  thus  undergoing  change, 
are  termed  by  Rutherford  metabolons. 

THE  TRANSFORMATIONS  OF  THE  RADIOACTIVE  ELEMENTS 
DIFFER  FUNDAMENTALLY  FROM  ORDINARY  CHEMICAL 
REACTIONS 

The  question  that  would  at  first  arise  is  this:  Are  the 
changes  that  are  going  on  in  the  radioactive  elements  funda- 
mentally different  from  chemical  reactions?  New  sub- 
stances with  different  properties  from  the  original  substances 
are  being  formed.  Energy  in  the  form  of  heat  is  liberated, 
and  these  changes  are  characteristic  of  ordinary  chemical 
transformations.  If  we  study  more  closely  the  changes 
that  are  taking  place  in  radioactive  matter,  we  shall  find 
marked  differences  between  them  and  chemical  reactions, 
as  has  already  been  pointed  out. 

In  the  first  place,  the  changes  in  radioactive  matter  take 
place  at  a  definite  rate,  which  is  entirely  unaffected  by  con- 
ditions. We  have  studied  a  number  of  such  radioactive 
changes  which  go  on  at  the  same  rate  at  the  temperature 
of  liquid  air  as  at  a  red-heat.  This  alone  would  show  that 
the  transformations  in  radioactive  matter  are  fundamentally 
different  from  chemical  reactions.  The  latter,  as  we  well 
know,  are  greatly  affected  by  conditions,  and  especially 
by  temperature.  The  velocity  of  chemical  reactions  is  in 
general  greatly  increased  by  rise  in  temperature,  and  at 
very  low  temperatures  becomes  extremely  small  or  entirely 
vanishes. 


1 78        THE  ELECTRICAL  NATURE  OF  MATTER 

Again,  the  velocity  with  which  radioactive  changes  take 
place  is  very  small.  The  amount  of  uranium  transformed 
into  uranium  X,  or  thorium  into  thorium  X,  in  considerable 
intervals  of  time,  is  very  small  indeed. 

The  slowness  of  the  transformations  that  we  have  just 
been  considering  explains  why  such  elements  as  thorium, 
uranium,  and  the  like  still  exist,  and  have  not  all  been  trans- 
formed into  their  decomposition  products.  It  is  calculated 
that  at  least  thousands  of  years  would  be  required  for  enough 
thorium  to  be  transformed  into  thorium  X,  in  order  that  the 
transformation  would  be  detectable  by  the  most  sensitive 
balance.  Even  radium  yields  the  emanation  very  slowly. 
In  fact,  so  slowly  that  the  loss  of  a  particles  cannot  even 
be  weighed  until  larger  amounts  of  radium  are  obtainable. 
It  has  been  calculated  that  radium  will  transform  half  of 
itself  in  about  1,500  years.  The  loss,  therefore,  can  scarcely 
be  detected  during  the  time  over  which  measurements  of 
radioactivity  have  thus  far  been  extended. 

That  radium  is,  however,  undergoing  decomposition  is 
certain,  and  if  it  were  not  being  produced  in  some  way  all 
of  the  radium  now  in  existence  would  eventually  disappear. 
That  radium  is  being  continually  produced,  probably  from 
uranium,  will  be  shown  in  the  next  chapter. 

Another  marked  difference  between  the  transformations 
that  are  taking  place  in  radioactive  matter  and  chemical 
reactions  is  in  the  amount  of  energy  set  free.  We  have 
already  become  familiar  with  the  fact  that  radium  liberates 
quantities  of  energy  incomparably  greater  than  any  other 
known  substance.  If  we  compare  the  amount  of  heat  set 
free  when  the  most  vigorous  chemical  reactions  take  place, 
with  that  liberated  by  salts  of  radium,  the  former  is  utterly 
insignificant. 

We  must,   therefore,   abandon  any  attempt   to   explain 


THEORETICAL  CONSIDERATIONS  179 

the  transformations  of  the  radioactive  elements  on  the 
basis  of  chemical  reactions.  The  two  processes  take  place 
according  to  different  laws.  They  are  affected  differently 
by  change  of  conditions.  They  yield  different  products, 
considered  both  from  the  standpoint  of  matter  and  of  energy. 
In  a  word,  they  are  fundamentally  different  processes. 

It  is  one  thing  to  point  out  that  radioactive  processes  are 
not  chemical  reactions;  it  is  a  different  matter  to  find  out 
the  nature  of  the  transformations  that  are  taking  place  in 
radioactive  substances.  That  we  have  a  satisfactory  sugges- 
tion to  account  for  these  transformations  will  now  appear. 

THE   ELECTRON  THEORY   OF   J.    J.    THOMSON  AS   APPLIED  TO 
RADIOACTIVITY 

It  would  be  difficult  to  account  for  the  instability  of  the 
chemical  atom  on  the  older  theory  that  a  chemical  atom  is  a 
homogeneous,  indivisible  unit.  In  terms  of  the  modern 
theory  of  the  atom  advanced  by  J.  J.  Thomson,  we  can 
readily  see  how  an  atom  could  be  unstable  and  send  off 
particles,  just  like  the  radioactive  atoms  do. 

In  terms  of  the  theory  of  Thomson,  which  we  have  called 
the  electron  theory,  a  chemical  atom,  as  we  saw  in  earlier 
chapters,  is  made  up  of  a  fairly  large  number  of  electrons 
or  negative  electrical  charges,  moving  within  a  sphere  of  uni- 
form, positive  electrification.  The  particles  are  held  in 
their  relative  positions  by  their  mutual  repulsions,  and  the 
attraction  of  the  positive  electricity.  The  heavier  atoms 
contain  a  larger  number  of  electrons  than  the  lighter  atoms 

-  the  approximate  number  in  any  atom  being  expressed 
by  the  atomic  weight  of  that  atom  in  terms  of  hydrogen  as 
unity,  multiplied  by  770. 

We  can  easily  conceive  of  some  of  the  electrons,  in  their 
rapid  movement,  coming  into  such  a  position  that  they 


l8o        THE  ELECTRICAL  NATURE  OF  MATTER 

would  escape  and  fly  off  from  the  atom  into  space.  This 
would  be  especially  the  case  with  the  heavier  atoms,  which 
represent  very  complex  systems  of  electrons.  From  such 
highly  complicated  systems  we  might  expect  a  more  or  less 
constant  escape  of  such  particles. 

Again,  we  might  not  only  expect  individual  electrons  to 
escape  from  the  atom,  but  groups  oj  electrons.  Indeed, 
groups  of  these  negative  electrical  charges  would  be  more 
likely  to  escape  from  the  atom  than  single  charges. 

The  facts  of  radioactivity  are  in  perfect  accord  with  the 
above  conclusions.  It  is  the  atoms  with  largest  mass  that 
are  radioactive.  Thorium  has  an  atomic  weight  of  232.5, 
uranium  of  238.5  and  radium  either  225,  or  more  probably 
in  the  neighborhood  of  256  or  258.  No  radioactive  sub- 
stance is  known  having  a  small  atomic  weight,  and  all  of 
the  heaviest  atoms  are  radioactive. 

In  the  earlier  stages  of  radioactive  change  it  is  the  a 
particles  that  are  shot  off.  The  a  particles  have  a  mass 
probably  about  four  times  that  of  the  hydrogen  atom. 
This  means  that  they  are  helium  atoms  (atomic  weight  4), 
that  are  shot  off  from  the  radioactive  atom.  The  atom, 
having  lost  this  comparatively  large  helium  atom,  is  dif- 
ferent in  nature  from  the  original  atom.  The  system 
is  not  yet  stable,  and  another  a  particle  or  atom  of  helium 
is  shot  off,  and  another  condition  of  the  radioactive  mat- 
ter produced.  This  may  continue  through  several  stages, 
until  after  a  while  the  individual  electrons  begin  to  come 
off  as  the  ft  particles.  It  will  be  remembered  that  the  y 
rays  are  set  up  where  the  ft  rays  impinge  upon  solid 
matter. 

Thus  we  see  that  the  theory  of  matter  advanced  by  J.  J. 
Thomson,  and  which  was  developed  at  some  length  in  the 
earlier  chapters,  enables  us  to  account  rationally  for  many 


THEORETICAL   CONSIDERATIONS  l8l 

of  those  remarkable  phenomena  that  we  have  studied  under 
the  general  head  of  radioactivity.  Further,  it  is  the  only 
theory  that  has  thus  far  been  proposed,  which  enables  us 
to  deal  at  all  satisfactorily  with  the  unstable  atom. 

IS  MATTER  IN   GENERAL  UNDERGOING  TRANSFORMATION? 

The  raising  of  such  a  question  would,  until  a  few  years 
ago,  have  been  regarded  as  extraordinary,  since  the  ele- 
ments were  regarded  as  stable  and  unchanging.  In  the 
light  of  the  recent  investigations  with  the  radioactive  ele- 
ments^ it  is  most  pertinent.  There  is  some  evidence,  as  we 
shall  see,  that  many  of  the  elements  are  radioactive  to  a  very 
slight  extent.  If  this  should  be  proved  to  be  due  to  the 
elements  themselves,  to  be  a  property  inherent  in  all  matter, 
and  not  caused  by  the  deposition  of  some  form  of  radio- 
active matter,  then,  from  what  has  been  said  above,  we 
must  regard  matter  in  general  as  undergoing  change.  This 
change  is  slow,  very  slow,  but  is  progressing  continuously; 
the  more  complex,  unstable  forms,  breaking  down  into 
simpler  aggregates  of  electrons. 

In  discussing  the  question  as  to  whether  matter  in  gen- 
eral is  radioactive,  the  following  consideration  must  be 
taken  into  account  as  Rutherford  has  shown. 

Since  the  a  particles  are  shot  off  from  radioactive  matter 
with  velocities  that  are  only  about  thirty  per  cent,  above 
the  critical  velocity,  i.e.,  the  velocity  necessary  to  affect 
a  photographic  plate,  to  produce  phosphorescence,  or  to 
ionize  a  gas,  and  thus  lead  to  the  detection  of  the  a  par- 
ticles; it  suggests  the  possibility  that  matter  in  general  may 
be  undergoing  a  disintegration  similar  to  the  radioactive 
elements,  but  that  the  a  particles  are  shot  off  with  a  velocity 
below  the  critical  and  therefore  escape  detection. 

It  is  probable  that  in  some  of  the  transformations  of  the 


1 82       THE  ELECTRICAL  NATURE  OF  MATTER 

radioactive  elements  which  were  thought  to  be  rayless,  a 
particles  are  actually  given  off,  but  with  a  velocity  that  is 
below  the  critical  and  they  therefore  are  not  detected. 

This  suggests  the  further  thought  that  all  matter  may 
really  be  radioactive.  Only  those  elements  that  shoot  off  a 
particles  with  velocities  above  the  critical  would  produce 
appreciable  ionization  in  a  gas,  and  thus  be  classed  as  radio- 
active, in  terms  of  our  present  methods  of  detecting  radio- 
activity. 

If  it  should  be  shown  that  all  matter  is  slightly  radio- 
active, then  we  should  be  forced  to  the  conclusion  of  the 
general  instability  of  the  chemical  elements.  However 
this  may  prove  to  be,  enough  has  already  been  established 
to  show  that  our  former  conceptions  of  the  nature  of  the 
chemical  element  must  be  fundamentally  modified. 


CHAPTER  XVII 

WIDE  DISTRIBUTION  OF  RADIOACTIVE  MATTER  AND  THE 
ORIGIN  OF  RADIUM 

THE  most  strongly  radioactive  substances  —  radium, 
actinium,  polonium  —  apparently  occur  in  very  small 
quantities.  Even  the  more  feebly  radioactive  elements, 
thorium  and  uranium,  are  not  among  the  more  common 
chemical  elements. 

A  question  of  very  great  importance  in  connection  with 
the  study  of  radioactivity  is  this:  Is  radioactive  matter 
small  in  quantity  and  confined  to  a  few  sets  of  conditions, 
or  is  it  widely  distributed?  The  fact  that  it  exists  in  any 
one  locality,  or  in  any  one  mineral  only  in  small  quantity, 
does  not  throw  much  light  on  the  question  of  the  scope  of 
its  distribution. 

We  shall  review  very  briefly  some  of  our  knowledge  of 
the  distribution  of  radioactive  matter,  as  far  as  our  globe 
is  concerned. 

RADIOACTIVE    MATTER    IN    THE    EARTH    AND    SEA 

It  has  been  shown  by  Elster  and  Geitel *  that  air  confined 
in  spaces  in  contact  with  the  earth,  such  as  certain  caves, 
becomes  radioactive.  The  same  result  was  obtained,  and 
to  a  more  marked  extent,  by  taking  air  from  some  depth 
below  the  surface  of  the  soil  by  means  of  a  pump.  Such 
air  contained  sufficient  quantity  of  the  radium  emanation 
to  induce  radioactivity  upon  the  walls  of  the  containing 

1  Phys.  Zeit.,  3,  574  (1902). 
183 


1 84       THE  ELECTRICAL  NATURE  OF  MATTER 

vessel,  especially  if  it  was  charged  negatively.  The  radio- 
activity decayed  at  such  a  rate  as  to  leave  no  question  that 
it  was  produced  by  the  radium  emanation.  These  phe- 
nomena were  shown  to  be  due  to  the  presence  of  radium 
in  the  ground,  which  diffused  into  the  air;  since  air  confined 
by  itself  in  a  metal  vessel,  away  from  contact  with  the  soil, 
did  not  become  radioactive. 

Similar  results  have  been  obtained  by  others,  so  that 
there  is  now  no  reasonable  doubt  that  the  radioactivity  of 
air  in  confined  spaces  is  due  to  the  presence  of  the  radium 
emanation,  which  gradually  diffuses  from  the  ground. 

It  was  shown  by  Ebert  that  air  which  is  radioactive, 
loses  its  radioactivity  when  passed  through  a  tube  sur- 
rounded by  liquid  air.  It  will  be  remembered  that  Ruther- 
ford, by  this  means,  condensed  the  emanation  from  radium, 
and  obtained  it  in  the  liquid  condition.  This  is  another 
bit  of  evidence  that  goes  to  show  that  the  radioactivity  of 
the  air  in  contact  with  the  earth  is  due  to  the  radium  emana- 
tion. 

The  amount  of  radioactive  matter  in  the  soil  seems  to 
vary  greatly  from  place  to  place.  Clay  soil  seems  to  be 
the  most  radioactive,  but  sandy  soils  are  not  infrequently 
radioactive. 

Carbon  dioxide  that  came  from  great  depths  in  the  earth 
was  found  to  be  radioactive.  It  lost  its  radioactivity  on 
standing  for  some  days. 

A  quite  appreciable  quantity  of  radioactive  matter  has 
been  found  in  certain  waters  that  percolate  through  the 
soil,  and  especially  in  those  that  come  from  considerable 
depths.  J.  J.  Thomson  1  has  shown  that  the  tap-water  of 
Cambridge,  England,  contains  radioactive  matter,  while 
the  waters  from  certain  deep  wells  in  other  parts  of  Eng- 

1  Phil.  Mag.,  4,  356  (1902). 


WIDE  DISTRIBUTION  OF  RADIOACTIVE  MATTER        185 

land  were  found  to  contain  quite  appreciable  quantities  of 
the  highly  radioactive  emanation.  This  emanation  decayed 
at  such  a  rate,  as  compared  with  the  emanation  from  radium, 
as  to  show  that  the  two  were  identical. 

Similar  results  were  obtained  by  Bumstead  and  Wheeler  1 
with  the  waters  at  New  Haven. 

One  of  the  most  interesting  results  of  this  character 
has  been  found  in  connection  with  certain  hot  springs, 
such  as  at  Bath,  in  England.  The  water  of  this  spring, 
which  comes  from  great  depths,  is  slightly  radioactive,  but 
the  mud  deposited  from  the  water  is  strongly  radioactive, 
due  to  the  presence  of  the  radium  emanation. 

It  is  also  a  matter  of  importance  that  in  the  gases  that 
escape  from  this  spring,  helium  has  been  jound.  This 
helium  comes,  almost  beyond  question,  from  the  decom- 
posing radium  emanation,  and  shows  that  radium  exists 
at  great  depths  beneath  the  surface  of  the  earth. 

The  simultaneous  occurrence  of  these  two  elements, 
and  the  fact  that  helium  is  produced  from  the  radium  ema- 
nation, lead  us  to  suspect  the  presence  of  radium  wherever 
helium  is  found  —  as  in  the  sun. 

Quite  recently  Joly,  in  his  admirable  little  book  on  "  Ra- 
dioactivity and  Geology,"  has  shown  that  there  is  an  enor- 
mous amount  of  radium  in  the  waters  of  the  sea,  and  es- 
pecially in  the  deposits  on  the  sea  floor.  He  shows  that  the 
total  quantity  of  radium  in  sea  water  is  about  twenty 
thousand  tons,  and  that  in  the  deposits  under  the  sea  there 
are  more  than  a  million  tons  of  radium. 

RADIOACTIVE  MATTER  IN  THE  AIR 

It  has  been  known  for  some  time  that  a  charged  body 
surrounded  by  air  may  lose  its  charge  rather  more  rapidly 

1  Amer.  Journ.  Sci.,  17,  97  (1904). 


1 86        THE  ELECTRICAL  NATURE  OF  MATTER 

than  can  be  accounted  for  by  the  leak  through  the  sup- 
ports. This  would  indicate  that  the  air  is  ionized  to  some 
extent. 

The  cause  of  this  ionization  remained  for  a  long  time 
unknown,  and,  indeed,  has  only  recently  been  discovered. 
After  the  discovery  of  radium  and  its  comparatively  wide 
distribution,  it  occurred  to  Elster  and  Geitel  that  radium 
might  be  present  in  small  quantity  in  the  air,  and  if  so, 
this  would  account  for  the  ionization  and  conductivity  of 
the  air.  They  undertook  to  test  the  atmospheric  air  for  the 
presence  of  radioactive  matter,  and  in  the  following  manner. 

It  had  already  been  shown  by  Rutherford  that  a  nega- 
tively charged  wire,  suspended  in  the  presence  of  the  emana- 
tion from  radium  or  thorium,  would  collect  upon  it  the 
radioactive  decomposition  products  of  the  emanation. 
Elster  and  Geitel,1  utilizing  this  fact,  exposed  a  long  wire 
charged  to  a  high  negative  potential  to  the  air,  and  then 
tested  it  for  the  presence  of  radioactive  matter. 

After  the  wire  had  been  thus  exposed  for  several  hours, 
it  was  placed  in  a  closed  vessel  with  a  charged  electroscope. 
The  latter  was  discharged  much  more  rapidly  than  nor- 
mally, showing  the  presence  of  radioactive  matter  upon  the 
wire,  which  ionized  the  gas  around  the  electroscope. 

The  presence  of  radioactive  matter  upon  the  wire  was 
further  shown  by  rubbing  the  wire  with  a  piece  of  drying 
paper  that  had  been  dipped  in  hydrochloric  acid.  The 
paper  became  quite  strongly  radioactive.  When  a  long, 
negatively  charged  wire  was  suspended  in  air  that  had 
remained  undisturbed  for  some  time  in  contact  with  the 
earth,  as  in  certain  caves  Geitel 2  showed  that  enough 
radioactive  matter  was  deposited  upon  the  wire,  which, 

1  Phys.  Zeit.,  2,  590  (1901). 
»  Ibid.,  3,  76  (1901). 


WIDE  DISTRIBUTION  OF   RADIOACTIVE  MATTER        187 

when  removed  by  a  piece  of  leather  moistened  with  am- 
monia, produced  a  visible  phosphorescence  in  barium  plati- 
nocyanide  when  the  salt  was  brought  near  to  it.  This 
radioactive  matter  also  exerted  an  action  on  a  photographic 
plate,  and  photographs  were  obtained  by  Geitel  by  means 
of  it. 

The  same  experimenter  studied  the  rate  at  which  the 
radioactive  matter  upon  the  negatively  charged  wire  under- 
went decay.  It  was  found  to  decay  like  the  radioactive 
matter  deposited  from  the  radium  emanation. 

If  the  wire  was  charged  positively  no  radioactive  matter 
was  deposited  upon  it.  Since  the  radioactive  matter  was 
drawn  to,  and  deposited  upon  a  negatively  charged  wire, 
and  not  upon  a  positive  wire,  we  must  conclude  that  the 
radioactive  matter  in  the  air  is  charged  positively. 

All  of  these  facts  point  to  one  conclusion.  The  radioactive 
matter  in  the  air  comes  from  the  radium  emanation.  This 
shows  that  radium  emanation  is  present  in  the  atmosphere. 

The  amount  of  radium  emanation  in  the  air  varies  greatly 
in  different  localities.  In  certain  cases  the  radioactivity 
of  the  air  is  relatively  great,  as  has  already  been  stated. 
The  amount  of  radium  emanation  in  the  air  in  some  locali- 
ties is  more  than  a  dozen  times  as  great  as  in  other  regions. 
Certain  experiments  made  in  northern  Norway  would 
seem  to  show  an  abnormally  great  amount  of  radium  emana- 
tion in  the  air  in  that  region. 

Since  the  radium  emanation  in  the  air  probably  comes 
from  radium  in  the  soil,  the  amount  of  the  emanation  in  the 
air  in  any  large  locality  may  be  taken  as  a  rough  index  of 
the  amount  of  radium  in  the  soil  in  that  locality.  This  is, 
of  course,  only  an  approximate  relation,  unless  frequently 
repeated  tests  were  made,  since  the  winds  shift  the  air  so 
frequently  from  one  region  to  another. 


1 88        THE  ELECTRICAL  NATURE  OF  MATTER 

Elster  and  Geitel l  found  that  the  radioactivity  of  the  air 
not  only  changed  from  one  locality  to  another,  but  was  not 
constant  in  any  given  locality.  It  varied  with  a  number  of 
conditions.  On  cold,  frosty  mornings  the  activity  was 
unusually  high.  The  lower  the  barometer  the  greater  the 
induced  radioactivity  in  the  air  in  any  given  region.  This 
is  just  what  would  be  expected  if  the  radioactive  matter 
in  the  air  came  from  radium  in  the  earth.  The  radium 
emanation,  being  a  gas,  diffuses  from  the  earth  in  which 
it  is  formed  from  the  radium  present  there,  into  the  at- 
mosphere. The  lower  the  barometric  pressure  the  more 
emanation  will  pass  out  of  the  fissures  and  fine  pores  in  the 
earth  into  the  atmosphere.  Since  the  radioactive  matter 
in  the  air  comes  from  the  radium  emanation,  the  lower  the 
barometer  the  more  radioactive  matter  present  in  the  air. 

All  of  these  facts  point  to  the  same  conclusion,  which  is 
that  already  stated,  that  the  air  contains  a  form  of  radio- 
active matter.  This  conclusion  is  still  further  confirmed 
by  the  following  facts: 

If  the  air  contains  radioactive  matter,  we  might  expect 
that  some  of  it  would  be  carried  along  with  objects  moving 
through  it. 

Fortunately  the  means  for  testing  this  conclusion  are 
supplied  to  us  by  nature.  When  drops  of  rain  or  flakes  of 
snow  fall  through  the  atmosphere,  they  might  be  expected  to 
carry  down  with  them  some  of  the  radioactive  matter  in  the 
air.  This  has  been  tested  by  C.  T.  R.  Wilson  2  in  England, 
and  in  the  case  of  snow  by  Allan 3  in  Canada.  Wilson 
found  that  freshly  fallen  rain  showed  the  presence  of  quite 
an  appreciable  amount  of  radioactive  matter.  This  radio- 
activity, however,  rapidly  decayed. 

1  Phys.  Zeit.,  4,  522  (1903). 

2  Cam.  Phil.  Soc.  Proc.,  n,  428;  12,  17  and  85  (1902-1903). 

3  Phys.  Rev.,  16,  237,  306  (1903). 


WIDE  DISTRIBUTION  OF  RADIOACTIVE  MATTER        189 

If  barium  chloride  is  added  to  freshly  fallen  rain,  and 
the  barium  precipitated  by  sulphuric  acid,  the  barium 
sulphate  that  is  thrown  down  is  quite  radioactive,  showing 
that  the  radioactive  matter  in  the  water  is  carried  down 
with  the  precipitate. 

Both  Wilson  and  Allan  found  that  newly  fallen  snow 
was  radioactive.  When  a  considerable  quantity  of  the 
snow  was  melted  and  the  resulting  water  evaporated,  a 
radioactive  residue  was  left  behind. 

The  radioactivity,  however,  rapidly  decayed,  as  in  the 
case  with  the  freshly  fallen  rain.  All  of  the  above  facts 
taken  together  leave  no  reasonable  doubt  as  to  the  presence 
of  radioactive  matter  in  the  air. 

IS  MATTER  IN  GENERAL  RADIOACTIVE? 

Having  found  a  number  of  chemical  elements  that  are 
radioactive,  and  having  shown  that  these  are  radioactive  to 
such  different  degrees,  the  question  naturally  arises,  Are  there 
not  other  substances  that  possess  radioactivity?  It  is  possible 
that  there  may  be  a  large  number  of  the  chemical  elements 
that  are  feebly  radioactive,  or  all  matter  might  be  radioac- 
tive to  some  slight  extent,  as  has  already  been  mentioned. 

The  first  experiments  bearing  upon  the  broad  question 
were  those  of  Mme.  Curie,  and  these  gave  negative  results. 
She  examined  a  large  number  of  the  chemical  elements 
for  radioactivity,  and  found  it  manifested  only  by  those 
already  considered.  The  question  in  this  connection  is 
whether  the  method  employed  by  Mme.  Curie  was  suffi- 
ciently sensitive. 

An  exactly  opposite  result  has  since  been  obtained  by  a 
number  of  investigators,  and  especially  by  Strutt.1  It 
seems  now  to  be  fairly  well  established  that  some  forms  of 
1  Phil.  Mag.,  5,  680  (1903). 


IQO  THE  ELECTRICAL  NATURE   OF  MATTER 

ordinary  matter  are  radioactive  to  a  very  slight  extent, 
but  unquestionably  radioactive. 

Campbell  and  Wood  x  have  found  that  potassium  and 
rubidium  salts  give  out  ionizing  rays  which  resemble  the 
)8  rays  of  uranium,  and  they  conclude  from  their  work 
that  potassium  has  about  one-thousandth  the  radio- 
activity of  uranium.  These  results  have  been  confirmed 
by  the  work  of  Levin  and  Ruer.2 

McClennan  and  Kennedy 3  investigated  a  large  number 
of  potassium  salts  and  found  that  they  were  all  radio- 
active. Calcium  and  rubidium  salts  were  slightly  radio- 
active, while  salts  of  lithium,  radium,  and  ammonium 
showed  no  radioactivity. 

Strong4  found  that  various  salts  of  potassium,  rubidium 
and  erbium  are  radioactive. 

It  has  been  pointed  out  that  subatomic  changes  might 
go  on  in  the  atoms  and  a-like  particles  be  expelled  with 
velocities  too  small  to  be  detected  by  the  usual  methods; 
i.e.,  with  velocities  below  the  critical.  In  such  cases  it  would 
be  expected  that  the  minerals  of  these  elements  would 
contain  accumulated  helium.  Strutt5  examined  a  large 
number  of  minerals  and  found  that  all  of  the  helium  pres- 
ent could  be  accounted  for  by  the  radium  uranium  and 
thorium  in  them  (beryl  being,  however,  an  exception). 
This  would  argue  against  the  radioactivity  of  matter  in 
general. 

THE  ORIGIN  OF  RADIUM 

We  have  seen  that  radium  is  unstable,  undergoing  con- 
tinual decomposition.     From  the  rate  at  which  radium 
1  Proceed.  Phil.  Soc.,  Camb.,  14,  I,  15  (1907). 
2Phys.  Zeit,  April  15  (1908). 

3  Phil.  Mag.,  Sept.  (1908). 

4  Amer.  Chem.  Journ.,  42,  147  (1909). 

5  Proceed.  Roy.  Soc.,  A,  80,  572. 


THE   ORIGIN   OF   RADIUM  IQI 

is  decomposing,  it  has  been  pointed  out  by  Rutherford  that 
if  the  whole  earth  were  pure  radium,  a  few  thousand  years 
hence  it  would  have  only  the  radioactivity  of  pitchblende. 

Since  many  of  the  minerals  that  contain  radium  have 
existed  much  longer  than  the  above  period,  it  is  obvious 
that  radium  must  be  produced  from  something,  or  all  of 
the  radium  would  long  since  have  disappeared.  The  inter- 
esting and  important  problem  is,  then,  to  find  out  what 
is  the  source  of  radium;  from  what  substance  or  substances 
it  is  produced. 

Since  radium  occurs  in  uranium  minerals,  it  was  early 
suspected  by  Rutherford  that  radium  might  be  produced 
from  uranium. 

Soddy,  working  with  Rutherford,  took  up  this  problem 
and  published  his  results  in  I9O4.1  A  kilogram  of  uranium 
nitrate  was  freed  from  radium  until  it  contained  less  than 
io-13  grams,  which  was  the  smallest  quantity  that  could  be 
detected  by  the  electroscope.  The  uranium  nitrate  was 
then  allowed  to  stand  for  twelve  months,  and  was  tested 
again  for  radium.  Soddy  points  out  that  the  presence  of 
radium  in  the  laboratory  renders  the  electroscopes  in- 
capable of  detecting  such  minute  traces  of  radium  as  they 
otherwise  could  do.  He,  however,  feels  justified  in  stating 
that  the  amount  of  radium  in  the  kilogram  of  uranium 
nitrate,  after  it  had  stood  for  a  year,  was  less  than  io-11 
grams.  Soddy  concludes  that  this  settles  the  question  as 
far  as  the  production  of  radium  from  uranium  is  concerned. 
Uranium  cannot  be  regarded  as  the  parent  of  radium, 
since  from  the  above  result,  if  any  radium  is  produced 
from  uranium,  less  than  one  ten-thousandth  of  the  theo- 
retical quantity  necessary  to  maintain  the  present  condi- 
tion of  equilibrium  is  produced. 

xNat.,  70,  30  (1904). 


IQ2  THE   ELECTRICAL   NATURE   OF   MATTER 

Soddy  recognizes  that  if  substances  intermediate  between 
uranium  and  radium  were  formed,  his  result  could  be 
explained.  He,  however,  thinks  that  such  assumptions 
are  not  justified. 

Just  a  week  prior  to  the  publication  of  the  paper  by 
Soddy  in  Nature,  a  short  article  appeared  in  the  same 
journal  by  Whetham,1  in  which  he  stated  that  he  had 
examined  several  specimens  of  uranium  compounds,  which 
had  been  preserved  in  the  laboratory  from  seventeen  to 
twenty-five  years.  A  larger  amount  of  radium  emanation 
was  obtained  from  these  old  specimens  than  from  more 
recently  prepared  samples  of  these  same  uranium  com- 
pounds. 

This  observation  was,  to  say  the  least,  suggestive,  and 
made  it  highly  desirable  that  more  work  should  be  done 
along  this  same  line. 

About  this  time  a  suggestion  was  made  by  Joly,2  which 
is  well  worthy  of  serious  consideration.  Joly  suggested 
that  instead  of  radium  being  a  disintegration  product  of 
uranium  or  thorium,  it  may  be  produced  by  the  interaction 
of  some  of  the  radioactive  substances  with  the  non-radio- 
active constituents  of  pitchblende.  Radium  would  then 
be  a  product  of  synthesis  from  simpler  things. 

This  suggestion  of  Joly  is  especially  important  if  it  should 
be  shown  that  the  atomic  weight  of  radium  is  greater  than 
that  of  thorium  or  uranium.  We  should  naturally  expect 
these  substances,  in  breaking  down,  to  yield  products  with 
smaller  atomic  weights  than  their  own.  If  radium  has  a 
larger  atomic  weight  than  either  of  these  radioactive  ele- 
ments, it  is  a  little  difficult  to  see  just  how  it  could  be  formed 
as  the  direct  result  of  their  disintegration.  It  might,  how- 

1  Nat.,  70,  5  (1904). 

2  Ibid.,  70,  80  (1904). 


THE   ORIGIN   OF   RADIUM  1 93 

ever,  be  produced  by  the  recombination  of  certain  of  the 
decomposition  products  of  these  elements  with  one  another, 
or,  as  Joly  suggests,  by  the  combination  of  these  with  other 
substances  occurring  in  the  pitchblende. 

Some  light  has  been  thrown  by  McCoy  l  on  the  possible 
origin  of  radium.  He  pointed  out  that  if  radium  is  a 
decomposition  product  of  uranium,  all  uranium  minerals 
must  contain  radium,  and  in  quantities  proportional  to 
the  amounts  of  uranium  in  the  minerals.  Since  all  in- 
termediate products,  such  as  uranium  X,  the  radium 
emanation,  etc.,  are  present  in  these  minerals  in  quanti- 
ties proportional  to  the  total  amounts  of  uranium,  it 
follows  that  the  total  radioactivity  of  every  natural  uranium 
ore  is  proportional  to  the  amount  of  uranium  contained 
in  it. 

McCoy  analyzed  a  number  of  uranium  ores  from  different 
localities,  and  determined  their  radioactivities  by  means 
of  the  electrical  method.  He  found  that  the  activity  of  all 
uranium  ores,  which  did  not  contain  appreciable  quantities 
of  thorium,  was  directly  proportional  to  the  amount  of 
uranium  contained  in  them.  In  other  words,  the  radio- 
activity of  any  given  quantity  of  uranium  ore,  divided 
by  the  percentage  of  uranium  contained  in  it,  is  a  constant. 
This  constant  was  termed  the  activity  coefficient. 

It  was  further  shown  that  the  radioactivity  of  chemi- 
cally prepared  uranium  compounds  is  directly  proportional 
to  the  amount  of  uranium  contained  in  them.  Such  com- 
pounds also  have  a  constant  activity  coefficient. 

More  elaborate  experiments  on  this  same  problem  have 
been  made  by  Boltwood,2  who  arrived,  however,  at  essen- 

1  Ber.  d.  deutsch.  chem.  Gesell.,  37,  2641  (1904). 

2Amer.  Journ.  Sci.,  18,  97  (1904);  Phil.  Mag.,  9,  599  (1905);  Nat.,  70, 
80  (1904). 


194  THE   ELECTRICAL   NATURE   OF   MATTER 

tially  the  same  result.  The  amount  of  radium  contained 
in  the  uranium  minerals  was  determined  by  measuring 
electrically  the  emanation  that  is  given  off  when  a  weighed 
quantity  of  the  mineral  is  dissolved  or  decomposed,  and  the 
solution  boiled  or  allowed  to  stand  in  connection  with  a 
closed  glass  vessel.  We  can  measure  the  activity  of  the 
emanation  very  accurately,  and  this  furnishes  us  with  a 
reliable  means  of  measuring  the  amount  of  radium  in  a 
given  substance  if  there  are  no  other  emanating  sub- 
stances present.  If  we  simply  wish  to  determine  the  rela- 
tive amounts  of  radium  in  any  two  substances,  it  is  only 
necessary  to  measure  the  activity  of  the  emanation  pro- 
duced by  equal  weights  of  these  substances. 

Boltwood  used  an  improved  method  for  analyzing  the 
uranium  minerals,  which  is  obviously  very  important. 
The  results  that  he  obtained  for  somewhat  more  than 
twenty  uranium  minerals  are  quite  satisfactory,  pointing 
conclusively  to  the  proportionality  between  the  amount 
of  radium  in  the  mineral  and  the  amount  of  uranium 
present. 

To  give  a  more  exact  idea  as  to  the  meaning  of  this  rela- 
tion, Boltwood  divided  the  amount  of  radium  in  the  mineral 
by  the  amount  of  uranium,  to  see  whether  the  ratio  would 
be  constant  for  the  different  minerals.  The  author  con- 
cludes that  from  his  results  there  is  direct  proportionality 
between  the  quantity  of  uranium  and  the  quantity  of  radium 
in  the  minerals,  and  that  radium  is  formed  from  uranium. 

He  points  out  that  certain  of  the  methods  that  have  been 
used  for  determining  uranium  quantitatively  are  defective, 
which  is  obviously  a  matter  of  the  greatest  importance  in 
the  present  connection. 

Experiments  similar  to  those  of  Soddy  were  carried  out 
by  Boltwood,  to  see  whether  radium  is  produced  directly 


THE   ORIGIN  OF   RADIUM  IQ5 

from  uranium.  He  comes  to  the  same  conclusion  as  Soddy, 
-  that  it  is  not.  He  agrees  with  the  suggestion  of  Ruther- 
ford, that  probably  one  or  more  intermediate  products 
exist  between  the  uranium  atom  and  the  radium  atom. 
Such  products,  however,  have  not  yet  been  discovered, 
unless  the  suggestion  of  Rutherford,  that  possibly  actinium 
is  such  a  product,  is  correct. 

In  a  quite  recent  paper,  Rutherford  and  Boltwood l 
point  out  that  as  the  amount  of  radium  in  uranium  minerals 
is  proportional  to  the  amount  of  uranium  present  in  those 
minerals,  the  amount  of  radium  to  the  gram  of  uranium  in 
the  mineral  should,  of  course,  be  a  constant.  The  value 
of  this  constant  can  easily  be  calculated,  if  the  relative 
radioactivity  of  pure  uranium  and  pure  radium  is  known. 
To  determine  the  amount  of  radium  occurring  in  the  mineral 
with  say  one  gram  of  uranium,  they  compared  the  radio- 
activity of  the  emanation  from  the  standard  amount  of 
pure  radium  bromide,  with  that  from  the  mineral  contain- 
ing a  known  quantity  of  uranium. 

They  found  that  the  amount  of  radium  to  one  gram  of 
uranium  in  uranium  minerals  is  about  7.4X10"'  grams. 
One  part  of  radium,  therefore,  occurs  with  about  1,350,000 
parts  of  uranium. 

From  these  data  it  is  easy  to  calculate  "the  amount  of 
radium  occurring  in  uranium  ores.  They  find  that  in  a 
ton  of  pitchblende  containing  sixty  per  cent,  of  uranium, 
which  is  a  rich  uranium  ore,  there  is  about  0.4  gram  of 
radium.  Lower  grades  of  pitchblende,  which  contain  less 
uranium,  will  contain  proportionally  less  radium. 

Boltwood  also  took  up  the  earlier  work  of  Soddy,  in 
which  the  latter  came  to  the  conclusion  that  radium  is  not 
formed  from  uranium,  because  uranium  nitrate  which  had 

1  Amer.  Journ.,  Sci.,  20,  55  (1905). 


196  THE  ELECTRICAL  NATURE   OF  MATTER 

stood  for  a  year  or  so  did  not  contain  any  appreciable  quan- 
tity of  radium. 

He  l  repeated  the  experiment  of  Soddy  and  obtained 
similar  results.  A  comparatively  large  quantity  of  ura- 
nium nitrate  was  carefully  purified  by  recrystallization. 
One  hundred  grams  were  dissolved  in  water  and  the  solu- 
tion sealed  up  in  a  bulb.  After  standing  thirty  days  the 
bulb  was  opened  and  all  gases  removed  from  the  solution 
by  boiling.  All  the  gases  removed  from  the  solution  were 
brought  in  contact  with  an  electroscope.  It  was  found 
that  the  amount  of  radium  present  in  the  uranium  at  the 
start  was  less  than  i.yXio"11  grams.  The  uranium  solu- 
tion was  again  sealed  up  in  the  bulb  and  allowed  to  remain 
for  six  months.  The  amount  of  radium  present  was  again 
tested  and  found  to  be  less  than  5.7Xio~n  grams. 

After  390  days  the  test  was  repeated,  and  with  the  same 
result;  the  amount  of  radium  present  in  the  solution  still 
being  less  than  i.yXio"11  grams.  If  any  radium  was 
formed  from  the  uranium  during  this  period,  the  above 
results  show  that  less  than  one  sixteen-hundredth  of  the 
quantity  required  by  theory  was  produced. 

These  results  would  seem  to  show  pretty  conclusively 
that  radium  is  not  formed  directly  from  uranium.  The  work 
of  McCoy  and  Boltwood,  however,  establishes  a  propor- 
tionality between  the  amount  of  radium  in  uranium  ores, 
and  the  amount  of  uranium  contained  in  them.  Taking 
all  these  facts  into  account,  we  must  conclude  that  uranium 
is  the  parent  of  radium,  but  that  the  latter  is  not  formed  directly 
from  the  former.  One  or  more  intermediate  products  with  a 
slow  rate  of  change  must  be  formed.  These  on  breaking 
down  yield  radium  directly  or  indirectly. 

The  above  historical  treatment  of  the  attempts  to  dis- 

1  Amer.  Journ.  Sci.,  20,  239  (1905). 


THE  ORIGIN  OF  RADIUM  197 

cover  the  immediate  parent  of  radium  is  preserved  on 
account  of  its  historical  interest.  The  direct  parent  of 
radium  has  now  probably  been  discovered.  Boltwood  1 
announced  near  the  close  of  1906  that  he  had  obtained 
from  a  uranium  mineral  a  small  quantity  of  a  substance 
which  he  supposed  was  actinium,  and  in  which  the  amount 
of  radium  present  was  more  than  doubled  in  six  months. 
Rutherford 2  showed  that  this  substance  could  not  be 
actinium,  since  it  was  not  sufficiently  radioactive;  Ruther- 
ford3 a  little  later  succeeding  in  separating  from  his 
actinium  by  means  of  hydrogen  sulphate,  a  portion  which 
was  about  one  hundred  times  as  radioactive  as  the  actin- 
ium, showing  that  the  parent  of  radium  was  chemically 
quite  different  from  actinium,  being  more  readily  carried 
down  in  the  above  precipitation. 

Radioactinium,  the  cause  of  the  activity  of  actinium,  is 
more  concentrated  in  the  precipitate  formed  as  described 
above,  doubling  the  activity  in  twenty  days,  but  this  was 
shown  not  to  be  due  to  the  growth  of  radium.  Boltwood 4 
showed  that  his  new  substance  did  not  produce  actinium 
emanation,  nor  actinium  X,  nor  did  the  above  precipita- 
tion carry  down  any  radioactinium  from  a  preparation 
that  had  stood  for  five  months. 

The  rate  at  which  the  new  substance  produced  radium 
was  constant  for  five  hundred  days,  which  shows  that  its 
rate  of  decay  is  slow,  being  about  twelve  years.  It  is  pro- 
duced from  uranium  X  at  a  correspondingly  slow  rate. 

1  Nat.,  75,  p.  54. 

2  Ibid.,  75,  270. 

3  Nat.,  76,  126.     Phil.  Mag.,  14,  733. 

4  Nat.,  Sept.  26  (1907).    Amer.  Journ.  Sci.,  24,  370. 


198  THE   ELECTRICAL   NATURE   OF   MATTER 

IONIUM 

Boltwood  called  this  substance  ionium  from  its  ionizing 
power.  Chemically  it  is  closely  allied  to  thorium,  while 
actinium  more  closely  resembles  lanthanum.  No  method 
has  yet  been  discovered  for  separating  it  from  thorium. 

Ionium  gives  out  a  rays  of  small  penetrating  power. 
They  have  a  range  of  only  2.8  centimetres  of  air.  The  /3 
radiations  if  any  are  present,  are  also  easily  absorbed.  Its 
activity  is  only  about  0.7  of  that  of  radium  in  equilibrium. 
It  gives  off  no  emanation. 

Marckwald  and  Keetman l  confirm  all  of  the  observations 
made  by  Boltwood.  They  obtained  the  ionium  from  the 
pitchblende  in  the  following  manner:  The  mineral  was 
dissolved  in  nitric  acid,  the  nitrates  converted  into  sul- 
phates by  means  of  sulphuric  acid.  The  sulphates  of  lead, 
barium,  and  radium  were  filtered  off.  Hydrofluoric  acid 
was  then  added  and  this  gave  a  precipitate  of  the  fluorides 
of  cerium,  yttrium,  and  thorium.  These  were  dissolved  in 
sulphuric  acid  and  the  thorium  precipitated  by  means  of 
oxalic  acid.  Ionium  resembling  thorium  so  closely  in  its  prop- 
erties is  precipitated  along  with  the  thorium,  from  which 
no  method  has  thus  far  been  devised  for  separating  it. 

Hahn 2  reached  the  same  conclusion  in  an  entirely  dif- 
ferent way.  He  found  that  commercial  thorium  salts 
contain  considerable  quantities  of  radium,  although  the 
thorium  came  from  monazite  sand  which  contains  only  a 
little  uranium.  The  amount  of  radium  in  the  thorium  salt 
increased  with  the  age  of  the  salt.  The  amount  of  radium 
in  a  freshly  prepared  specimen  of  thorium  nitrate  was 
found  to  double  in  two  months. 

1  Ber.  d.  deutsch.  chem.  Gesell.,  41,  49  (1908). 

2  Ibid.,  40,  4415. 


THE   ORIGIN   OF   RADIUM 


IQQ 


Taking  into  account  all  of  the  above  evidence,  there 
seems  to  be  no  reasonable  doubt  that  ionium  exists  and 
is  the  direct  parent  of  radium. 


THE    COMPLETE    SERIES    OF    TRANSFORMATIONS    IN     WHICH 
RADIUM  IS   INVOLVED 

The  complete  series  of  transformations,  starting  with 
uranium  and  ending  with  lead,  is  given  in  the  following 
table.  The  kind  of  radiations  given  off  at  the  stage  of  the 
transformation  is  also  shown: 


Name 

Time  of  half  decay 

Kind  of  rays 

Uranium 

6  X  io9  years 

a 

t 

Uranium  X 

24.6  days 

ft,y 

Uranium  Y 

1.5  days 

ft 

Ionium 

? 

a 

* 

Radium 

2000  years 

S  ft 

Emanation 

3.85  days 

a 

Radium  A 

3  minutes 

a 

f 

Radium  B 

26.8  minutes 

ft,y 

t 

Radium  C  >•  p  * 

19.5  minutes 
1.4  minutes 

a,  ft, 

Radium   D  ) 
Radio-Lead  ) 

16.5  years 

ft    ' 

f 

Radium  E 

5  days 

ft,y 

t 

Radium  F  ) 

t 

136  days 

a 

Polonium   ) 

200  THE   ELECTRICAL   NATURE    OF   MATTER 

EMANIUM 

During  the  last  year  or  two  a  number  of  articles  have 
appeared  on  a  supposedly  new  radioactive  substance  called 
emanium.  It  was  discovered  in  pitchblende  by  Giesel, 
and  was  found  to  be  related  chemically  to  the  elements  of 
the  cerite  group,  and  especially  to  lanthanum  and  cerium. 

The  dehydrated  chloride  or  bromide  shows  a  discontinu- 
ous phosphorescent  spectrum  of  three  lines.  Glass  in  which 
the  substance  was  preserved  for  some  months  was  colored 
violet.  Paper  was  browned  and  decomposed.  After  the 
maximum  activity  was  reached  the  activity  of  the  solid 
substance  underwent  no  further  change.  Giesel 1  con- 
cluded in  his  earlier  work  that  this  substance  is  a  new  radio- 
active element.  He  thought  that  the  results  could  not  be 
accounted  for  as  due  to  any  induced  activity  resulting  from 
contact  with  radium.  When  a  current  of  air  was  blown 
over  the  preparation  of  the  supposedly  new  substance  and 
then  against  a  phosphorescent  screen,  bright  scintillations 
or  sparks  made  their  appearance,  that  were  more  distinct 
and  larger  than  in  the  case  of  radium,  and  the  effect  was 
more  striking  than  in  the  ordinary  spinthariscope. 

This  strongly  radioactive  substance,  supposed  by  Giesel  to 
be  a  new  radioactive  element,  was  named  by  him  emanium. 

He,  however,  pointed  out  about  a  year  ago  that  it  was 
possible  that  emanium  was  identical  with  the  actinium 
discovered  by  Debierne.  At  that  time,  however,  there  was 
not  sufficient  known  about  the  properties  of  the  two  sub- 
stances to  determine  whether  they  were  identical  or  not. 

Debierne  undertook  a  comparative  study  of  actinium 
and  emanium,  and  concluded  that  the  two  were  identical. 
Giesel,  however,  points  out  that  there  are  certain  differences 

1  Ber.  d.  deutsch.  chem.  Gesell.,  37,  1696  and  3963  (1904);  38,  775  (1905). 


THE    ORIGIN   OF   RADIUM  2OI 

in  the  properties  of  the  two  substances  that  need  explana- 
tion before  we  can  regard  the  two  as  identical.  The  induced 
radioactivity  produced  by  emanium  falls  to  half-value  hi 
34.4  minutes,  while  that  of  actinium  requires  40  minutes  to 
decay  to  half -value. 

He  also  points  out  that  the  three  lines  observed  in  the 
phosphorescent  spectrum  of  emanium,  having  the  wave- 
lengths 4885.4,  5300,  and  5909,  respectively,  had  not  at  that 
time  been  observed  in  actinium.  Further,  since  these 
lines  could  not  be  identified  with  those  of  any  known  ele- 
ment, it  seemed  fair  to  conclude  that  they  were  due  to  a 
new  element. 

Giesel  studied  the  activity  of  emanium,  and  showed  that 
the  emanation  was  not  driven  out  by  heating  or  solution  as 
with  radium,  and  concluded  that  there  was  a  solid,  non- 
volatile substance  formed. 

Subsequent  work,  however,  has  shown  that  the  three 
lines  mentioned  above  are  really  not  new. lines  at  all,  which 
can  be  referred  to  a  new  element,  but  were  produced  by 
one  of  the  didymia  that  was  present.  This  invalidates  one 
of  the  lines  of  reasoning  which  led  Giesel  to  conclude  that 
he  was  dealing  with  a  new  substance.  He  separated  the 
active  constituent  or  constituents  from  emanium,  in  a  man- 
ner analogous  to  that  employed  by  Rutherford  in  the  case 
of  thorium.  He  found  most  of  the  activity  of  the  emanium 
in  the  small  residue  that  remained  when  the  solutions 
containing  emanium  were  precipitated  with  ammonia.  On 
account  of  the  analogy  with  thorium  X,  Giesel  termed 
this  active  residue  emanium  X. 

He  showed  further  that  when  emanium  X  has  been  sepa- 
rated from  emanium,  more  emanium  X  is  continually  being 
formed.  This  again  is  strictly  analogous  to  the  condition  of 
things  in  thorium.  It  was  also  established  that  most  of  the 


2O2        THE  ELECTRICAL  NATURE  OF  MATTER 

activity  of  emanium  is  due  to  the  emanium  X  that  is  present 
in  it. 

The  question  as  to  the  identity  of  emanium  and  actinium 
was  taken  up  quite  recently  by  Hahn  and  Sackur.1  It  will 
be  recalled  that  the  argument  advanced  by  Giesel  based 
upon  spectrum  analysis,  in  favor  of  the  two  substances 
being  different,  had  been  shown  to  be  untenable  —  the  lines 
supposed  by  him  to  be  produced  by  emanium  being  really 
those  of  one  of  the  didymia. 

The  second  argument  advanced  by  Giesel  to  show  that 
these  two  substances  are  different  was  based  upon  the 
different  amounts  of  time  required  for  the  induced  radio- 
activities produced  by  the  two  substances  to  decay  to  half 
their  initial  value.  These  measurements  have  been  repeated 
by  Hahn  and  Sackur,  with  the  result  that  the  amounts  of 
time  required  in  the  two  cases  are  the  same. 

These  authors  have  also  determined  the  amount  of  time 
required  for  the  emanation  itself  from  the  two  substances  to 
decay  to  half-value.  They  find  that  the  time  in  the  two 
cases  is  exactly  the  same,  to  within  the  limits  of  experimental 
error. 

From  these  facts  they  conclude  that  the  actinium  of 
Debierne  and  the  emanium  of  Giesel  are  probably  identical. 

New  light  seems  to  have  been  thrown  on  the  relation 
between  actinium  and  emanium  by  Marckwald.2  He 
thinks  that  he  has  satisfactorily  solved  the  problem.  The 
rare  earths  obtained  from  the  radium  mother-liquor  were 
transformed  into  chlorides,  and  the  thorium  precipitated 
by  thiosulphate.  This  thorium  showed  strong  emanating 
power  and  contained  the  actinium  of  Debierne.  From  the 
solution  cerium  was  first  precipitated,  and  then  the  didymia 

1  Ber.  d.  deutsch.  chem.  Gesell.,  38,  1943  (1905). 
*Ibid.,  38,  2264  (1905). 


THE   ORIGIN   OF   RADIUM  203 

and  lanthanum  as  oxalates,  which  were  transformed  into 
oxides.  Neither  the  cerium  nor  the  mixture  of  the  didymia 
and  lanthanum  showed  any  considerable  emanating  power. 

The  thorium  was  then  purified  by  subjecting  it  to  a  num- 
ber of  processes,  but  the  emanating  substance  clung  to  the 
thorium  in  all  of  these  operations. 

The  activity  of  this  actinium  which  accompanied  the 
thorium  was  studied  for  several  months  and  was  found  to 
decrease.  The  mixture  of  the  didymia  and  lanthanum, 
on  the  contrary,  acquired  greater  and  greater  emanating 
power  with  time  —  their  emanating  power  increasing  in  the 
same  ratio  as  that  of  the  actinium  in  the  thorium  decreased. 
The  author  points  out  that  this  is  analogous  to  the  case  of 
thorium  and  thorium  X. 

The  explanation  of  these  facts  seems  very  simple.  The 
radioactive  substance  that  accompanies  the  lanthanum 
gives  off  no  emanation.  It,  however,  decomposes  into  a 
second  substance,  which  in  its  chemical  reactions  resembles 
thorium.  When  the  latter  substance  undergoes  further 
decomposition  a  strong  emanation  results. 

To  test  the  correctness  of  this  interpretation  the  follow- 
ing experiment  was  performed:  A  half-gram  of  pure 
thorium  oxide  was  added  to  eighteen  grams  of  the  didymia- 
lanthanum  mixture,  which  had  stood  until  it  emanated 
strongly.  The  whole  was  then  dissolved  in  hydrochloric 
acid  and  the  thorium  again  precipitated  by  thiosulphate. 
The  thorium  precipitated  now  contained  nearly  all  the 
emanating  power;  the  solution  of  the  didymia-lanthanum 
mixture  contained  very  little  of  the  emanation. 

The  conclusion  seems  necessary  that  there  is  something 
in  the  didymia-lanthanum  mixture  which  yields  a  sub- 
stance closely  allied  chemically  to  thorium,  and  which  has 
strong  emanating  power. 


204  THE   ELECTRICAL   NATURE    OF    MATTER 

Emanium  and  actinium  are,  then,  not  identical.  Ema- 
nium  undergoes  decomposition  and  yields  actinium  —  ema- 
nium  is  the  parent  of  actinium. 

ATOMIC  WEIGHTS   OF  RADIOACTIVE   LEAD  FROM 
DIFFERENT   SOURCES 

Richards  and  Lembert 1  have  obtained  radioactive  lead 
from  a  number  of  different  sources;  have  purified  these 
materials,  and  have  determined  their  atomic  weights  by 
the  same  methods.  They  obtained  the  following  results: 

At.  Wt. 

Lead  from  North  Carolina  uraninite  206.40 

Lead  from  Joachimsthal  pitchblende  206.57 

Lead  from  Colorado  carnotite  206.59 

Lead  from  Ceylonese  thorianite  206.82 

Lead  from  English  pitchblende  206.86 

Common  lead  207 . 1 5 

All  of  the  radioactive  specimens  of  lead  had  a  lower 
atomic  weight  than  ordinary  lead.  The  atomic  weight 
was  found  not  to  be  proportional  to  the  radioactivity  of  the 
lead  in  question. 

The  ultraviolet  spectrum  of  radioactive  lead  was  shown 
to  be  identical  with  that  of  ordinary  lead. 

"The  inference  seems  to  be  that  radioactive  lead  con- 
tains an  admixture  of  some  substance  different  from  or- 
dinary lead,  and  very  difficult  to  separate  from  it  by 
chemical  means." 

This  substance  either  has  the  same  spectrum  as  lead,  or 
no  spectrum  in  the  ultraviolet  where  lead  has  a  spectrum, 
or  its  spectrum  is  masked  by  lead. 

The  atomic  weights  of  a  number  of  other  elements  from 
different  sources,  were  determined.  These  include  copper, 
1  Journ.  Amer.  Chem.  Soc.,  36,  1329  (1914). 


THE    ORIGIN    OF   RADIUM  205 

silver,  iron,  sodium  and  chlorine.  The  atomic  weight  of 
each  of  these  was  found  to  be  constant,  independent  of  the 
source  from  which  the  element  came. 

CONCLUSION 

The  investigations,  of  which  a  general  account  has  been 
given  in  these  chapters,  mark  a  new  epoch  in  the  develop- 
ment of  the  physical  sciences.  Some  of  the  results  obtained 
are  as  important  from  the  standpoint  of  the  physical  chemist 
as  from  that  of  the  physicist.  Facts  have  been  brought  to 
light  which  are  of  a  character  that  are  very  different  from 
anything  hitherto  known.  The  existence  of  extremely  pene- 
trating forms  of  radiation,  the  instability  of  the  chemical 
atom,  the  formation  of  one  elementary  substance  from  another, 
the  existence  of  a  form  of  matter  that  can  charge  itself  elec- 
trically, that  can  light  itself,  and  that  can  give  out  an  amount 
of  heat  that  is  almost  inconceivably  great,  are  some  of  the 
facts  to  which  we  must  now  adapt  ourselves. 

These  are  magnificent  developments  with  which  to  open 
the  new  century.  Probably  still  more  surprising  facts  are 
awaiting  men  of  science  before  its  close.  It  seems  not  too 
much  to  predict  that  as  the  nineteenth  century  surpassed 
the  preceding  eighteen  in  the  development  of  scientific 
knowledge  and  the  discovery  of  truth,  just  so  the  twentieth 
century  will  exceed  them  all  in  the  gifts  of  pure  science 
to  the  store  of  human  knowledge.  The  wave  of  scientific 
investigation  for  its  own  sake,  that  has  recently  swept  over 
the  entire  civilized  earth,  must  yield  a  rich  harvest  to  those 
who  shall  be  permitted  to  reap  it. 


INDEX 


Actinium,  51,  65. 

decomposition  products  of,  156. 
X,  158. 

Allan,  radioactivity  of  the  air,  188. 
Alpha  particle,  action  on  a  photo- 
graphic and  on  a  fluorescent 
plate,  83. 

particle,  ratio  of  —  for  the,  74. 
m 

particles  are  probably  helium 

atoms,  81. 
particles,  critical  velocity  of  the, 

80. 
particles  produce  delta  particles, 

81. 

particles  stopped  by  matter,  84. 
rays,  70,  72,  96. 
Anions  and  cations  in  terms  of  the 

electron  theory,  36. 
Atomic  weight  of  radium,  57. 

Atomic  weights  of  radioactive  lead, 

204. 
Atom,  nature  of  in   terms  of   the 

electron  theory,  29,  35,  40. 
not  the  same  mass  as  the  ion,  37. 
Thomson's  conception  of  the,  31. 
Becquerel  ray,  44. 

rays,  properties  of,  47. 
Becquerel's  theory  of  spinthariscope, 

80. 
Beta  and  gamma  rays,  85. 

particle,  determination  of  —  for 
m 

the,  88. 

particle,  mass  of,  89. 

particles,  charge  carried  by,  86. 

rays,  70,  85,  96. 

rays  from  radium,  89. 
Bolt  wood,  on  the  discovery  of  ion- 
ium, 198. 

on  the  origin  of  radium,  193, 
195. 


Bragg  and  Kleeman,  alpha  particles 

stopped  by  matter,  84. 
and    Kleeman,    on    the    alpha 
particles  given  off  by  radium, 

Bumstead  and  Wheeler,  radioactive 

matter  in  tap-water  at  New 

Haven,  185. 
Bunsen    ice    calorimeter    used    to 

measure    heat    liberated    by 

radium  salts,  109. 

Campbell  and  Wood,  on  radio- 
activity of  potassium  and 
rubidium  salts,  190. 

Canal  rays,  15. 

Cathode  particle,  value  —  for  the,  5. 
m 

ray,  3- 

rays,  properties  of,  89. 
Cations  and  anions  in  terms  of  the 

electron  theory,  36. 
Charge  carried  by  beta  particles,  86. 
on  a  gaseous  ion,  comparison 
with  that  on  a  univalent  ion 
of  an  electrolyte,  13. 
to  the  mass  for  the  positive  ion, 

ratio  of,  15. 
to  the  mass  of  the  ion  in  a  gas 

ratio  of,  3. 

Chemical  effects  produced  by  radio- 
active substances,  101. 
reactions  differ  from  radioactive 

transformations,  177. 
Conducting  gas,  how  it  differs  from 

a  non-conducting,  2. 
Conductivity,  electrical,  of  gases,  i. 
of  dielectrics,  radium  increases 

the,  zoo. 

of  gases,  conditions  which  in- 
crease the,  i. 
Corpuscle,  15,  22. 
nature  of,  19. 


207 


208 


INDEX 


Critical  velocity  of  the  alpha  parti- 
cles, 80. 
Crookes,  an  cathode  rays,  4. 

on  the  spinthariscope,  78. 
Curie  and  Deslandres,  helium  pro- 
duced from  radium,  135. 

M.  and  Laborde,  show  that 
radium  produces  heat,  106. 

Mme.  and  M.  on  the  charge 
carried  by  beta  particles,  86. 

Mme.  and  M.,  radium  charges 
itself  positively,  88. 

Mme.  and  M.,  show  that  beta 
particles    carry    negative 
charges,  87. 

Mme.,  discovers  polonium,  50. 

Mme.,  discovers  radium,  49. 

Mme.,  luminosity  of  radium 
compounds,  98. 

Mme.,  on  polonium,  152. 

Mme.,  on  the  atomic  weight  of 
radium,  57,  62. 

Mme.,  "residual  activity,"  180. 

Mme.,  separated  radium  from 
pitchblende,  50. 

Mme.,  separates  radium,  51. 

M.,  studied  the  radiations  from 

radium,  70. 

Curies  discover  induced  radioactiv- 
ity, 140. 

on  radioactivity  of  matter  in 
general,  189. 

phosphorescence  produced  by 
radium  salts,  99. 

studied  chemical  effects  pro- 
duced by  radioactive  sub- 
stances, 101. 

Debierne    discovers    actinium,    51, 

65. 
on  emanium,  200. 

Decay    of    induced    radioactivity, 

143- 
of  the  emanation,  129. 

Decomposition   products   of   radio- 
active substances,  153. 

Delta  particles  produced  by  alpha 

particles,  81. 
rays,  70. 

Demarcay,  on  the  spectrum  of  ra- 
dium, 56. 

Dewar  and  M.  Curie  measured  heat 
liberated     by     radium,     by 


means    of    liquid    hydrogen, 

107. 
on  the  rate  at  which  helium  is 

produced  from  radium,  82. 
Dielectrics,    radium    increases    the 

conductivity  of,  100. 
Discovery  of  radium,  49. 
Distribution  of  radioactive  matter, 

184. 

Dobereiner  triads,  26. 
Duane,  on  the  alpha  particles,  81. 

Earth's  age,  effect  of  heat  liberated 
by  radium  on  the  calculated, 
114. 
Electrical  charge,  radium  produces 

on  itself  an,  117. 
conductivity  of  gases,  i. 
theory  of  matter,  19. 
Electrons,  groups  of,  180. 
Electron  theory  and  radioactivity, 

40. 
theory  and  the  Periodic  System, 

31- 
theory,  cations  and  anions  in 

terms  of  the,  36. 
theory,  nature  of  the  atom  in 

terms  of  the,  29,  35. 
the  ultimate  unit  of  matter,  23. 
Elster    and    Geitel    on    radioactive 

matter,  distribution  of,  183. 
and  Geitel,  radioactivity  of  the 

air,  186,  188. 
Emanation,  amount  of,  121. 

and  helium,  relation  between, 

136- 

decay  of,  129. 
deposits     radioactive     matter, 

properties    of    this    matter, 

145- 

discovered  by  Rutherford,  48. 
from     radioactive     substances, 

118. 

heat  evolved  by,  129,  130. 
heat  produced  by  the,  148. 
helium  produced  from,  130. 
helium  produced  from  the,  127. 
may    effect    transmutation    of 

certain  elements,  138. 
method  of  obtaining,  119. 
molecular  weight  of,  123. 
nature  of,  122. 
of  thorium,  119. 


INDEX 


209 


produces  induced  radioactivity, 
142. 

X,  147,  201. 

Emanating   power   of    radium,    re- 
covery of,  128. 
Emanium,  200,  201. 

Fluoroscopic  method  of  studying 
radioactivity,  67. 

Gamma  rays,  70,  93,  96. 

rays,  theory  as  to  the  nature  of, 

95- 
Gas  conducting,  how  it  differs  from 

a  non-conducting,  2. 
Gaseous  ions  produced  by  different 

means.  —  for  the,  8. 
m 

Gases,  conductivity  of,  conditions 
which  increase  the,  i . 

determination  of  the  mass  of 
the  negative  ion  in,  10. 

electrical  conductivity  of,  i. 

—  constant  for  different,  7. 
in 
Geiger  and  Rutherford,  counted  the 

alpha  particles,  82. 
Generalization,  importance  of,  170. 
Giesel  on  emanium,  200. 
on  emanium  X,  201. 
studied  the  action  of  the  mag- 
netic field  on  the  radiations 
from  radium,  69. 
Godlewsky,  on  actinium  X,  159. 
Goldstein  discovered  canal  rays,  15. 

Hahn,  on  the  origin  of  radium,  198. 
Heat  evolved  by  the  emanation,  129, 
130. 

given  off  by  radium,  effect  on 
the  calculation  of  the  age  of 
the  earth,  114. 

liberated  by  radium,  amount  of, 
no. 

liberated  by  radium,  calcula- 
tion of  the  amount  of,  117. 

liberated  by  radium  measured 
by  the  Bunsen  ice  calorim- 
eter, 109. 

liberated  by  radium,  measure- 
ment of  the,  106. 

produced  by  radium,  117. 

produced  by  radium  salts,  106. 


produced  by  radium,  source  of, 
in. 

produced    by    the    emanation, 
148. 

produced  by  radium,  theories 

as  to*  the  source  of,  115. 
Helium    and    emanation,    relation 
between,  136. 

atoms,  alpha  particles  are  prob- 
ably, 81. 

discovered  in  the  sun  by  Lock- 
yer,  on  the  earth  by  Ramsay, 
132. 

from  radium,  further  experi- 
ments, 134. 

produced  from  the  emanation, 
127,  130. 

Induced  radioactivity,  66,  140. 

radioactivity  due  to  deposit  of 

radioactive  matter,  144. 
radioactivity  produced  by  the 

emanation,  142. 
radioactivity  undergoes  decay, 

143- 

Ion  gaseous,  comparison  of  the 
charge  on,  with  that  on  a 
univalent  ion  of  an  elec- 
trolyte, 13. 

in  gases,  determination  of  the 
mass  of  the  negative,  10. 

not  the  same  mass  as  the  atom, 

37- 
positive,  ratio  of  the  charge  to 

the  mass  for  the,  15. 
Ionium,  198. 
lonization     method     of     studying 

radioactivity,  68. 
Ions  gaseous,  produced  by  different 

means,  —  for  the,  8. 

• 

of  electrolytes,  —  different  for 
m 

the  different,  7. 

Joly,  on  the  origin  of  radium,  192. 
radioactivity  and  geology,  184. 

Kaufmann,    on    the    velocity    and 
mass   of    the   beta   particles 
from  radium,  89. 
on  the  electrical  origin  of  mass, 
21. 


210 


INDEX 


Kaufmann,  on  the  velocity  of  the 

different  beta  particles  from 

radium,  20. 
Kleeman  and  Bragg,  on  the  alpha 

particles  given  off  by  radium, 

75- 

Laborde  and  M.  Curie  show  that 

radium  produces  heat,  106. 
Lead  radioactive,  atomic  weights  of, 

204. 

Lenard  rays,  8. 
Lockyer  first  discovered  helium  in 

the  sun,  132. 
Luminosity  of  radium  compounds, 

98. 
of  radium  salts,  117. 

Mackenzie,  on  the  ratio  —  for  the 
m 

alpha  particles,  76. 
Madsen,  on  the  gamma  rays  from 

radium,  95. 

Makower  determined  the  molecular 
weight  of  the  emanation,  125. 
on  stopping  of  beta  particles,  92. 
Marckwald  on  emanium,  202. 
on  polonium,  64. 
on  the  origin  of  radium,  198. 
Marignac,  on  Prout's  hypothesis,  23. 
Mass  of  an  ion  not  the  same  as  that 
of  the  atom  from  which  it 
is  formed,  37. 
of  beta  particles,  89. 
of  the  negative  ion  in  gases, 

determination  of  the,  10. 
of  the  positive  ion,  ratio  of  the 

charge  to  the,  15. 
to  charge  of  the  ion  in  a  gas, 

ratio  of,  3. 

Matter,  earlier  attempts  to  unify,  24. 
electrical  theory  of,  19. 
in    general    undergoing    trans- 
formation, 181. 
the  electron  the  ultimate  unit 

of,  23. 

McClelland   and    Hackett,   on   the 

stopping  of  beta  particles,  92. 

on  secondary  radiations,  92. 

McCoy,  on  the  origin  of  radium,  193. 

M.    Curie    and    Dewar    measured 

heat  liberated  by  radium,  by 

means  of  liquid  hydrogen,  107. 


Methods  used  in  studying  radio- 
activity, 66. 

Mendeleeff's  Periodic  System,  26. 

Meyer  Lothar,  Periodic  System,  26 

Molecular  weight  of  the  emanation, 
123. 

Negative  ion  in  gases,  determina- 
tion of  the  mass  of,  10. 
Newland's  Periodic  System,  26. 

Origin  of  radium,  183. 

Ostwald,  on  the  overthrow  of  scien- 
tific materialism,  23. 

Oxygen  transformed  into  ozone  by 
radium,  102. 

Ozone  produced  from  oxygen  by 
radium,  102. 

Particles,  total  number  shot  off  by 
radium,  97. 

Periodic  System,  electron  theory 
and  the,  31. 

Phosphorescence  produced  by  ra- 
dium salts,  99. 

Photographic  method  of  studying 
radioactivity,  67. 

Physiological  action  of  the  radia- 
tions from  radium,  104. 

Pitchblende,  radioactive  substances 

in,  63. 

radium  separated  from,  50. 
the  source  of  radium,  49. 

Polonium,  50,  63. 

action    of    on   a  photographic 
plate,  103. 

Positive  ion,  ratio  of  the  charge  to 
the  mass  of  the,  15. 

Precht  and  Runge  on  the  atomic 
weight  of  radium,  58. 

Properties  of  the  alpha,  beta,  and 
gamma  rays,  96. 

Prout's  hypothesis,  24. 

Radiation  from  thorium,  47. 
Radiations    from    radioactive    sub- 
stances, properties  of  the,  68. 
other  properties  of  the,  98. 
Radioactive  matter  deposited  by  the 
emanation,  properties  of,  145. 
matter,  distribution  of,  183. 
matter  in  earth  and  sea,  183. 
matter  produces  induced  radio- 
activity, 144. 


INDEX 


211 


substances  in  pitchblende,  63. 

substances  produce  chemical 
effects,  101. 

substances,  properties  of  the 
radiations  given  out  by,  68. 

transformations  differ  from 

chemical  reactions,  177. 
Radioactivity,  46. 

electron  theory  and,  40. 

induced,  66,  140. 

matter  in  the  air,  185. 

methods  used  in  studying,  66. 

emanation  in  the  air,  187. 

of  matter  in  general,  189. 

theory  of,  176. 

theory  of  Thomson,  179. 
Radiographs,  104. 
Radiotellurium,  64. 
Radiothorium,  154. 
Radium  A,  150. 

B,  150. 

C,  150. 

D,  151. 

E,  151. 

F,  151, 152. 

amount  of  heat  liberated  by, 
no. 

atomic  weight  of,  57. 

charges  itself  electrically,  117. 

complete  transformation  prod- 
ucts of,  199. 

compounds,  luminosity  of  the, 
98. 

decomposition  products,  152. 

discovery  of,  49. 

does  it  exist  in  the  sun,  113. 

emanation  may  effect  transmu- 
tation of  certain  elements,  138. 

facts  in  connection  with,  174. 

from  pitchblende,  separation  of, 

50- 

heat  produced  by,  effect  on 
source  of  solar  heat,  112. 

heats  itself,  117. 

increases  the  conductivity  of 
dielectries,  100. 

measurement  of  the  heat  liber- 
ated by,  106. 

origin  of,  183,  190. 

physiological  action  of  the 
radiations  from,  104. 

produces  ozone  from  oxygen, 
102. 


salts,  phosphorescence  produced 
by,  99. 

salts,  production  of  heat  by, 
106. 

self -luminosity  of,  117. 

separated  by  Mme.  Curie,  51. 

spectrum  of,  56. 

total  number  of  particles  shot 

off  by,  97. 

Ramsay  and  Soddy  measured  the 
amount  of  the  emanation,  121. 
Ramsay  first  discovered  helium  in 
the  air,  132. 

on  radiothorium,  154. 

results  with  radiothorium,  156. 

shows  that  helium  is  produced 
from  the  radium  emanation, 
132. 

shows  that  the  helium  emana- 
tion may  affect  certain  trans- 
mutations, 138. 

studied   the  properties  of   the 

emanation,  123. 
Ray,  Becquerel,  44. 

cathode,  3. 
Rays,  alpha,  72. 

X,4i. 

Regener  counted  the  alpha  particles, 
82. 

on   scintillations   produced   by 

beta  radiations,  92. 
Rowland,  on  the  nature  of  the  atom, 

36. 
Runge  and  Precht,  on  the  atomic 

weight  of  radium,  58. 
Russell,   on    fogging    of    a    photo- 
graphic plate  by  metals,  45. 
Rutherford   and   Barnes   measured 
heat  evolved  by  the  emana- 
tion, 130. 

and  Geiger  counted  the  alpha 
particles,  82. 

and  Miss  Brooks  determine  the 
molecular  weight  of  the  ema- 
nation, 124. 

and  Royds,  helium  from  alpha 
particles,  83. 

and  Soddy,  on  thorium  X,  165 
167. 

and  Soddy  study  the  emana- 
tion from  radium,  127. 

and  Soddy,  theory  of  radio- 
activity, 176. 


212 


INDEX 


Rutherford  calculated  the  amount  of 
heat  liberated  by  radium,  117. 

determines  the  ratio  of  —  for 

m 

the  alpha  particle,  74. 

discovers  the  emanation,  48, 
118. 

discovers  thorium  emanation, 
119. 

on  induced  radioactivity,  143. 

on  photographic  and  fluorescent 
action,  83. 

on  radium  E,  151. 

on  the  critical  velocity  of  the 
alpha  particles,  81. 

on  the  heat  produced  by  the 
radium  emanation,  148. 

showed  that  salts  of  thorium 
induce  radioactivity,  140. 

shows  that  the  alpha  particles 
are  complex,  75. 

studied  properties  of  the  active 
matter  deposited  by  the  ema- 
nation, 146. 

studied  the  action  of  the  mag- 
netic field  on  the  alpha  rays, 
72. 

studied  the  effect  of  low  tem- 
perature of  the  production  cf 
emanation,  128. 

studied  the  properties  of  the 
emanation,  122. 

Schmidt,  on  thorium  radiation,  47. 

Secondary  radiations  produced  by 
beta  rays,  92. 

Solar  heat  as  affected  by  heat  given 
off  by  radium,  112. 

Spectrum  of  radium,  56. 

Spinthariscope,  78. 

Becquerel's  theory  of  the  action 

of,  80. 
theory  of  the  action  of,  79. 

Stark,  on  positive  rays,  17. 

Strong,  on  radioactivity  of  potas- 
sium salts,  190. 

Strutt,  on  Prout's  hypothesis,  25. 
on  radioactivity  of  matter  in 
general,  189. 

Sun,  does  it  contain  radium,  113. 

Theory,  importance  of,  170. 

of  radioactive  phenomena,  176. 


Thomson,  J.  J.,  conception  of  the 

atom,  31. 
determination  of  the  mass  of 

the  negative  ion  in  gases,  10. 
on  positive  rays,  17. 
on    the    electrical    theory    of 

inertia,  19. 

on  the  ratio  of  —  for  different 

M 

gases,  7. 

on  the  value  of  —  for  the  cathode 
m 

particle,  5. 
radioactive  matter  in  tap-water 

of  England,  184. 
shows  that  the  alpha  particles 

are  charged  positively,  73. 
theory  of  radioactivity,  179. 
Thorium,  emanation,  119. 

facts  in  connection  with,  173. 
forms  radiaoctive  matter,  164. 
radiation,  47. 

recovers  radioactivity,  166,  168. 
X,  164. 

X,  decay  of  radioactivity,  165. 
X  from  thorium  emanation,  165 
Thorpe,  on   the   atomic   weight  of 

radium,  62. 

Transformation  of  matter  in  gen- 
eral, 181. 

products  of  radium,  199. 
Transmutation  of  the  elements,  not 
effected,  133. 

Unification   of   matter,    earlier   at- 
tempts to  affect  the,  24. 
Uranium  radioactive,  45. 

facts  in  connection  with,  171. 

recovery  of  activity  of,  162. 

X,  decay  of  activity  of,  162. 

X,  radiation  from,  163. 

Villard,  on  gamma  rays,  93. 

Watts,    on    the    atomic   weight    of 

radium,  58,  62. 
Wilson,  C.  T.  R.  condensation  of 

water-vapor  around  ions,  10. 
radioactivity  of  snow,  188. 

X  rays,  41. 

nature  of,  42,  48 


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